We're not retired but there are two of us. Our basic monthly bills are something like:
Food 300 euros
Phone/Internet: 75 euros
Insurances (House, car, health): 88 euros (basic mutuelle)
Gas: 110 euros (We have a fairly large, old, draughty house)
Electricity: 60 euros
Total: 633 euros
Water we pay twice yearly, I think. The last bill was 175 euros
We have a mortgage now which is relatively cheap compared to the one we have on our English home as interest rates are lower here (or were when we took it out and your mortgage stays at the rate it was on the day you took it out for the term of the mortgage). If you rent, that needs factoring in too.
Other costs are really down to the individual. Here in the Dordogne you can get lunch with wine (set menu, sometimes a couple of choices) for 12-15 euros a person. Petrol is about 1.29 euros/litre. You'll have some council tax equivalents to pay, taxe foncière and taxe d’habitation, although I think you can get reductions if on a low income.
i would say you could live on about 1,200 euros if you don't have a mortgage or rent to pay, but would need more to save for holidays, building repairs, updating, replacing electrical items, trips back to UK etc.