We have recently got a new shower that works off the combi. The heating comes on at 5.45am and when we go to have a shower at 6.45ish then the shower is hot for a sec then goes cold. If we have a shower later in the day then its fine. Help!

Not knowing the capacity of your boiler I would start the boiler a hour or do earlier to give the water time to heat up. Good luck, Tom