r.p.mcmurphy wrote:
I am converting a small bedroom into a wetroom with a high flow shower. i
plan to have two ho****er tanks to provide for the extra water required and
a solar water heater to help keep running cost down. i was also thinking
about trying to recover some of the heat lost down the plug hole by using a
heat exchanger just past the u-bend.

has anyone any experience of using one of these?

I couldn't find a UK source, in a few minutes of looking.
For a shower, the design I was thinkinf of trying was a simple ring in
the shower drain, past the strainer.
This makes the water drain in a hollow cylinder, which flows onto the
top of a coil of 15mm pipe.
The water then flows down round the inside and outside of the coil,
transferring heat as it goes (cold entry at bottom).

Shower drain is removable, and a brush can be used to clean off any
accumulation of stuff from the coil, if needed.

A flooded heat exchanger could probably be much more efficient, in the
same size, but suffers more from the likelyhood of clogs.