My 10 year old combi provides hot water eventually when all pipes en-route are warmed-up. Fills the bath in 1/2 hour unless you run cold at the same time when the hot stops then runs cold again for an eon.

Yea Yea I know its the run and lay of the pipes but that's the warning. Plan ahead and lag the hot water pipe and don't supply cold from the same pipe run or same side of any pressure reducing valve. This was put in by the house builder - nuff said.

Previous owners fitted an electric shower, thank goodness.