Thanks for all the replies and suggestions.
To answer some of the assorted queries:
It's probably scaled-up between the element bottle and the outlet, causing restricted flow. Is this shower connected to a gravity cold water tank in the loft as the cold supply, the traditional bathroom feed in Britain?
The unit is fed directly from the incoming mains supply. Although instant showers aren't my favorite units either for the reasons already stated, he opted for one mostly so that it could be used quickly during summer when the wood burner won't be providing a full cylinder of hot water all the time.
The flow rate seems to be quite adequate, and in fact pretty much the same as when fitted. Wouldn't restricted flow result in the water coming out hotter, anyway?
Those Triton water heaters only deliver cool water if they are located too far from the outlet. Was it delivering hot water previously ?
Yes, it's worked fine for 4 years or so since it was fitted. The heating element is right in the unit, so it comes straight out of the can into the shower hose.
When he said the water's now barely lukewarm, I think even that was an optimistic assessment. I think "Just about got the chill off it," would probably better describe it -- Warmer than the cold supply, but not by much.
The control valve does slow down the flow rate when turned to the hotter end, as it should.
Don't forget that the cold water supply is several degrees lower in winter and this is reflected directly in the output temperature. From my recollections you can't actually have much water flowing from these devices if you want it hot, and 7.5kW is a fairly modest rating for one.
I do wonder why they even bother to fit the high/low power switches on some of these, since the low setting is hardly ever going to be used by anyone.
If the temperature were down a little, I could chalk it up to an unusually cold supply, or something of that nature. But the fact that the water is so cold coupled with the elements still drawing 32+ amps of current just has me stumped.
I can feel the heat around the top of the can after switching off, and there's some heat coming back up onto the terminals from the elements, but that's about it.