Have had one of these

http://tinypic.com/r/dnf4le/9 (10.8Kw)

for the last 10+ years, used daily.

3 years ago or so the water temperature started to play up (would turn
cold suddenly when in use), and at the advice received here I cleaned
the filter on the water inlet, which fixed it, and no problems since.

Yesterday it started to misbehave again: under Medium or Low setting the
water is completely cold. When set to High it almost reaches acceptable
temperature when the dial is on max, but not hot enough. Anything below
that is pretty much cold.
Usual setting for this time of year is around 1, perhaps 2 o'clock.

I tried to clean the (very dirty) filter again, but it had no effect.

Any idea what to try next please?