How to figure out how much income to/can allocate to mortgage repayment?

Nine_Lives

The furniture thing isn't an issue as we have a family loan for that if needs be.

Yeah, there are no meters at my mums. Aside from the gas meter where you read it off & enter it into their website.

The difference with where we'll move to & my mums is that when we move, the house wont be occupied between 7:30am-6:00pm. My mums house is always occupied & therefore always costing something as there's always something being used. I suspect our electricity bill will be a fraction of my mums, but i don't know how much of a fraction.

Food is a toughie i suppose. Right now my mum does our tea for us 3 nights a week (she insists before anyone starts). We get the other 4. We buy the 3 meat for £10 from Sainsbury's, 2 packets of rice for £3, say a curry sauce is £1.80 ish. Some buns at 70p or 2 for £1.20. Then some chicken fillets for a few more quid. This will do up some chicken curry, turkey burgers (with spices we have at home already), chicken burgers or chicken with herbs & rice.
When we move though, we'll be looking more at buying in in 'bulk'. A sack of potatoes for e.g.. A bag of onions instead of an onion. Plenty of various veg that can all be thrown together to make something, which should have everything lasting longer.
Problem is, like the electricity bill (for e.g.) we wont know what that'll cost until we actually do it.