Sorry to bother again - had some good responses to question on venturi
showers and some comments about electric ones.

Off to see the house tonight and I have started to think Electric - mounted
in roof space. A neighbour has one - but it is a very expensive bells and
whistles one. The location of the heater unit seems to make good sense to me
as I don't need to think about neat piping or wiring. I could just tee off
the cold feed to the loft water tank and (hopefully) find a good route for a
suitable cable. (I would get an electrician to complete the wiring)
Question: Any suggestions for such a loft mounted electric shower? I haven't
seen any (apart from my neighbours) - but then they wouldn't look very
attractive on a display I suppose. Any ideas why the range seems so
limited - given that most bathrooms have roofspace above?