Why is my shower lukewarm in very cold months...i have a combi boiler with a single knob for temp control..?

Since the weather turned cold 4 weeks ago my shower has gone from warm to lukewarm at best.....its a single knob and non electrical with a combi boiler....i dont believe its a boiler issue as the hot water taps work elsewhere....i have also noticed that if i switch on the cold water in the bathroom basin, whilst I run the shower I am able to get hot water again....is this problem easy to fix...thanks...