Any question answered...

Is it an old boiler? We had a problem where the water would take a while to start to go hot, then get to scalding, then go cold, yo-yoing between the two forever. Was worse in summer, less common in winter.

Insides of the pipes had got all scaled and the thermostat was getting confused. It was a new boiler jobby.

Yeah, that's what it's doing (well, it doesn't get scalding, but as it starts to get hot/warm something ticks in the boiler and it gets freezing again. About a 10-15 second cycle). Heating is fine. Just the taps it seems. Happened twice in the last two days, but went away after turning heating off for a while. Assumed the cold spell was causing it to over heat.