I only get hot water when central heating is on and when heating is off no hot water?

i only get hot water when central heating is on and when heating is off no hot water! i dont have a water only switch like 99.9% of the people ive spoke to have! how do i get hot water in the summer without turning heating on long enough to have a shower! i have a megaflow in the airing cupboard and the boiler is a potterton, i think!