lukewarm water to my shower and bath but fine elshere in house?

Since the cold weather arrived, my new shower (Triton Mira, 2 levers, 1 for temp, the other for flow) and new bath (Beaumont taps & shower mixer), brought locally), recently installed by a professional plumber, are both passing lukewarm water. Elsewhere in the house the hot water is fine and the potterton combi boiler is approx 1 year old and seems to be working fine. The water pressure is ok at approx 2 constantly. We have quite a few rads in the house but even when I turn off the central heating there is no change. For 6 months both applicances have been working fine but the problem seems to have started since the temperatures have dropped outside. The distance from the boiler to both the shower & bath is quite short. If anyone can help give me a list of things that could be causing this that would be great.
Thanks in advance