@HolyCheeses, so what I think is possibly going on is a workaround on Mr Pre-nup's part whereby he's over-inflating the living costs and you'll not be paying into his mortgage at all, thus eradicating any future claim on his house you might have if you split up. (You've already confirmed he says it's a small mortgage, a ?remortgage for renovations.)
This is the spreadsheet so far:
food £1,400
cleaner £200 per month
council tax £150 £300 per month [you amended this]
sky £180
home insurance £100
dog walking £260
sewer waste water £30
boiler type coverage £30
garden maintenance £50
Total of above = £2,550
You said the total was approx £3,020 from memory.
We need to add in also for this large house the following (and these are conservative guesses):
energy costs (gas, electricity) circa £250
water costs (meter or set bill) circa 40
vet insurance and costs for dogs circa £50
cars / travel costs circa £200 minimum
All other stuff
The total's already way over £3,020. It's £3,090 and rising.
This means he doesn't (can't) actually have a mortgage on this house. I think he's going to take your money as a monthly bank transfer though, and make a 'profit' on it, so to speak, while letting you think you're paying into the mortgage and building up your ability to make a proportional future claim.
The alternative is that he has an even more bonkers scheme going on, or he's an idiot. But it all sounds mega dodgy and I'm glad you're out.