Lordy lord.
Just go down to Leroy or Brico or Casto and get the stuff you need. Build your bathroom and be done.
Arguing the toss over sizes and prices. Seriously anglo idiots !!!
I have been to B&Q in the UK and the stuff is quite frankly naff to be fair.
End the debate over France vs UK when it comes to Brico…pleeese !!
Well, yes, of course 
Although in my case it is the local Weldom where I am almost on first name terms with the staff!
Yes, we also have a macerator to a fosse toutes eaux - I have replaced the unit and it came with the one-way valve for the pipework.
Ours is in one of our gites and I would NOT recommend this, because children flushing the wrong things (eg. wet-wipes) can cause a lot of unpleasant work! - but in your own ensuite where you can treat it properly I don’t think you’ll have any problems.
As far as I am aware neither SPANC nor the marie have imposed any conditions on our proposed modernisation. This may be because the footprint is unaltered and the number of bedrooms unchanged. We are: bringing floor area into use, re-organising room use, hopefully adding more “salle d’eaux” functionality.
From research - SurviveFrance and others over the past two years the capacity of the fosse system is determined by the number of hypothetical occupants. If your proposed alterations are increasing the hypothetical occupants beyond the current capacity of your fosse system then I can understand SPANCs interest, One of the “opportunities” is the lack of consistency across France (as evidenced by contributions from subscribers in the different departments ). Sometimes e.g. micro-stations there is central legalative support that can help demonstrate that the project meets the required standards. In other cases it appears to be an exercise of “liberte” (I have little evidence of fraternity even between the indigenous population