I did a little maths around the average £35k salary. I went for lower end for costs e.g. Band C/D Council Tax, cheap car owned outright with typical daily commute of 30 miles round trip, no subscription TV/cheap phone with lower end package, being careful with energy but not freezing etc.
Assuming the minimum 5% into pension, take home is £2297pm.
From that subtract £200 council tax, £250 energy costs, you said tiny mortgage so say £700, House/car etc insurance £200, other car costs (fuel, tax, servicing, new parts like tyres, fuel) £280, connectivity (phone, broadband) £100. That takes you to just shy of £1750. With the remaining £540 a month you need food, clothing, household cleaning, any household maintenance, any additional petrol.
This is all assuming you have the most basic of everything possible - no iPhone, no Netflix or Sky, no eating out, no holidays of course, no HP on anything, any additional entertainment like cinema, birthday/Christmas gifts, activities/hobbies. So pretty much stripped down and sacrifices made.
OP try doing the maths on your own essential living costs, and you may get a shock regarding what's left over (or not)