Look for any device (shower, TMV, mixer tap) that mixes hot and cold water. Check the temperature of both inlet pipes (a finger will do). Run the tap-that-should-be-cold until it's coming out hot. Recheck the mixing devices, if one now has both inlet pipes hot that's the culprit. It needs fixing and non-return valves (NRVs) fitted.
Thanks for your reply, the cold water is getting up to 45 degrees Centigrade. There is a TMV which I have replaced but hasn't solved the problem. I don't have NRV's fitted. It seems from your reply I should try that next. It's not a new installation this problem has occurred recently.