£1000 a month on food for 5 adults sounds very reasonable - it's about £45-£50 per person per week.
My household is smaller than yours so this may not translate to your circs, but from experience, in planning to significantly reduce working hours, I have found the following most useful if it's at all feasible:
Build up a savings pot of 6 months to 1 year salary.
Keep a separate savings pot for food bulk buys - anything on offer that is part of your usual non-perishable shop, stock up on 3-6 months worth. Although this seems counter intuitive as it doesn't reduce your monthly outgoings initially, over the year it can develop into noticeable savings, and suppress the monthly food bill.
Stock up cupboards with long-life goods such as pasta, rice, tinned goods, and if you have a large freezer do the same with frozen foods - especially when on offer. Also if you're rural, collect and freeze free windfall cooking apples and blackberries if available for winter desserts.
For reducing the cost of meals - 2 or 3 times a week (perhaps at the weekend), serve a three course meal such as home made soup starter and hm soda bread, and home baked pudding that's filling. These elements are cheaper than the main part of the meal, so the main course can be smaller portions.
Ditch loyalty to just one supermarket if you have several available nearby, and scour 2 or 3 for offers online.
Most people are aware of yellow sticker reductions in large stores, but if you have a village or small town store, they will often have more basic items reduced such as eggs, meat, bacon, bread which can often be frozen. These frequently appear earlier in the day than the main supermarkets, so popping in to several, if they are within travelling distance on a morning can be a useful strategy.
Approach household finances with the mindset that it's a business which must make a profit, planning ahead for maximising income; and minimising costs and expenditure. Set aside a half day a week if possible, to do your accounts in a systematic way, balance the books, look for further savings and explore investment opportunities.
Good luck with your plans.