Saunier Duval Therma+ C25E 8.4 a 24,6kw
@AngelaR We have an old stone house. Power cut were a regular occurrences 15+ yrs ago, but rarely have them now, understand your point.
The flue leading out to the roof does not comply now, but the devis for a new boiler includes cost of deep cleaning the flue that is within the house, hope that makes sense.
Thank you all for the information, lots to think about while I wait for second opinions.
Saunier Duval are rebadged UK Glow-worm boilers (part of the Valliant Group).
One of my sons works for Glow Worm and has advised that low hot water pressure can be caused by a number of different issues such as faulty pressure sensor , leak in the system etc .
Wondered whether you have a fault code and know whether the boiler water pressure is between 1 and 1.5 bars?
BTW, I recently bought some sensors for my gas CH boiler.
Cost was around £25 each and fitting (by a gas engineer) took minutes.
A sensor is just a sensor, it may report a fault but the actual issue would be elsewhere. Yes could be the diverter valve internal leak as thats quite common.
I fully agree with @Corona exhaust pipe is nothing to do with it at all! It sounds like a mechanical problem to me. I suggest you call another heating engineer/ company. 8 yrs old isn’t old enough for it not to work and if anything needs replacing the parts should be available.
Thank you for the continuing suggestions, I will get the investigative team on the case, oh wait that’s me 
The hard part is finding a gas engineer.
Onwards and upwards
My neighbour had a problem with their oilfired boiler. Parts are no longer available and the chauffagist suggested that they convert their central heating system to an air-eau heat pump. With the various subsidies including low income (the owner has just retired), and our local area subsidy plus another the cost to him is around one third of the quoted installation cost. He’s definately going to switch and I’m waiting to see what effect it has on his overall energy costs.
A lot depends if your buildings are of similar construction and insulation levels otherwise its not much of a comparison to go by.