Sorry I've only just picked up your reply jmck.
I was wondering if it was mixer taps. A problem with a diverter valve will affect ALL hot outlets. Quite logical if you think about it. It's all 15mm pipework with a combi, so I'd rule out any flow problems.
I would say the mixer tap is faulty, allowing cold water to cross over to the hot side. Perhaps it's a "cartridge" type. The cartridge could need replacing. Either that, or treat yourself to some new taps.