Be careful with phrases like that @Peter_Goble will get arroused!
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Indeed you are right John if you are inferring that the risk of explosion is less with fioul vapour than with petrol vapour. However, the risk still exists in just the same way as there is risk of explosion even with such things as fine powders like flour for example. It just depends of the fioul vapour / air ratio being just right.
Iām right in the thick of thingsā¦ and loving every minuteā¦ if we lived further out, I would have to have a secret tunnel linking the house to the village centreā¦
The bigger danger by far is from leaked oil soaking into paper or material that would burnā¦ You are never going to get sufficient evaporation from heating oil to cause a volatile gas, the tanks are often kept in the same room as the burner after all. The fuel itself is safe, drop a lighted match into it and the match will be extinguished.
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The concrete blockwork looks fairly recent and soundly constructed. If you want to remove an exploratory block or two from the bottom, then I would suggest using a 10 or 12mm masonry drill to pierce holes through the mortar course every 20mm or so, and then a tap or two with a hammer and bolster should do the trick to break the remaining mortar and release the block. With a little care it should be possible to get the block out without breaking it. One of those āpatience is a virtueā jobs.
Seems like there is a 40mm plastic pipe coming from the top of the tank and going to the outside perhaps as an air vent ? Would be worth checking this for damage as well as the other pipework.
Certainly any cracked seals or joints on any of the pipework should be remedied.
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Can you get a good look at the base of the tank with a torch? Is there any spillage or is the floor clean?
I will go and take a look tomorrow. Will need to hoover up all the cigarette butts first. I do have one of those endoscope torch phone attachments but itās at my parents. I was trying to peer into the roof there to see what the loir were constructing! 
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Oo err missus! 
[quote=āMisty36140, post:38, topic:24019, full:trueā]when I raise it to dip in the tank it bends against the ceiling 
[/quote]
OOO ERRR MISSUS!! 
. That happens to mine!
If itās dry down there then you can probably rule out the tank being split, so then it will be a connection or the boiler itself I suppose (not an expert!).
First rule of troubleshooting - do the easiest thing first!
Second rule - donāt rely on the previous guy who was unable to fix it!
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You should be so lucky Peter, or rather the missus ! 
Iām āde-mob happyā , Ann, circa 1914-18, time to fade awayā¦? 
Donāt fade away too quickly Peter, whatever would we do without your aboundant crops and tall bendy bits !
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One further thought that has come to mind is that if the 40mm plastic pipe shown in the photos is indeed an air vent to the outside, then once outside, is the pipe just an open end or does it have some sort of anti wind pressure / anti insect device fitted to it ? If itās just an open end of a pipe then it could be that the wind is creating a pressure inside the oil tank and forcing fumes out through the cracked seal you mentioned. A couple of cheap 40mm plastic pipe bends glued in place so that the open end then faces down towards the ground may well help the situation.
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I believe that it is correct to say that every oil tank that is situated inside the building, or in a cellar or basement, should have a containment structure (enveloppe secondaire Ć©tanche) built around it that is capable of holding the entire contents of the tank in the case of leakage in order to comply with the environmental safety regulations about these things. https://www.legifrance.gouv.fr/affichTexte.do?cidTexte=JORFTEXT000000437731#LEGISCTA000020342700 Article 16 refers.
Our steel tank has such a wall around it that is only just over half as high as the tank as there is quite a distance between the wall and the tank itself.
According to the regulations, the room in which the tank is situated needs walls, floor, and ceiling which are fire resistant for half an hour, and there must be a fireproof outward opening door rated at 15 minutes minimum.
Like you I have seen many plastic tanks that donāt have a surrounding ācontainmentā structure, including one that is actually inside an upper area of the inhabited part of the house, which I have always thought of as rather risky.
To be honest I doubt that many tank installations actually comply with the regulations which came into force on July 1st 2004.
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Actually I think you may be failing to take account of the fact that the fumes / vapour given off by heating oil are in fact the lighter fractions of the oil, and therefore have different flashpoints than the liquid fuel itself. I agree that the risk of explosion is probably small, especially if the tank is situated in a cool environment, but it has to be accepted that escaping fumes from the tank headspace are more dangerous than the fuel itself, especially if those fumes congregate in an enclosed area.
Domestic heating oil is not much different from diesel fuel in itās make up, and certainly the latter can be explosive with as little as 1% vapour in the air.
Having been present at the scene of a number of gas / vapour explosions over the years it is not a risk that I would wish to take myself.
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I know what Iām saying. Gas is completely different.
Though I struggled to find definite reports of diesel vapour exploding in the situation described there are one or two which are close. Fairly extreme provocation to ignite seems to be a theme.
https://www.osha.gov/pls/imis/accidentsearch.accident_detail?id=68164.015
https://www.osha.gov/pls/imis/accidentsearch.accident_detail?id=202487260
https://www.osha.gov/pls/imis/accidentsearch.accident_detail?id=201623741
The technically minded may be interested in the following extract from a Material Safety Data Sheet (MSDS) from a reputable company:
General Fire Hazards
See Section 9 for Flammability Properties.
Vapors may be ignited rapidly when exposed to heat, spark, open flame or other source of ignition. When mixed with air and exposed to an ignition source, flammable vapors can burn in the open or explode in confined spaces. Being heavier than air, vapors may travel long distances to an ignition source and flash back. Runoff to sewer may cause fire or explosion hazard.
Source: https://www.hess.com/docs/eu-safety-data-sheets/no2fueloil_eu.pdf?sfvrsn=2