It's one of these fancy Boilermate systems - even with the water temp turned right up it's barely warm enough for a bath. If you leave it a while, the first 20 secs or so are a decent temp then it goes luke-warm again. Is this just a dodgy thermostat do you think?

Try here mate (http://www.hotwaterlimited.co.uk/boilermate2.htm)

Nice one - thanks :)

Tried adjusting that knob fully, storage heat set to max, and boiler on full whack but it's still no warmer.

Could it be the 'stat? If I use the immersion it doesn't get any warmer either.

Is it a boilermate II or 2000?

It's a Boilermate II - stupid bloody thing :)