On Tuesday, 13 August 2019 20:38:50 UTC+1, David WE Roberts (Google) wrote:
I was considering a hot water tank and an immersion heater for more total
heat over a slower period but that seems to run to over £200 for a basic
set up.

Unless you get a second hand one from someone upgrading to unvented hot water. Of course in a shed frost-proofing a header tank might be more difficult.

For hand washing there are what look like shower units without the shower
which makes me wonder if there is a way to divert the shower to the hand
basin and use the same unit.

Electric showers really aren't supposed to have any valve or possible restriction on the output, and some can be very fussy about the spray if not supplied by the manufacturer.

However multipoint electric instantaneous heaters are available.

https://www.stiebel-eltron.co.uk/en/products-solutions/dhw/instantaneous_waterheater/all-products.html?_charset_=utf-8&enabledFilterName=refcode_SF004051&enabledFilter Value=::|SF004051::x|SF004052::x|SF004053::x|SF004 054::x|SF004055::x|SF004056::x|SF004179::x|SF00418 0::x|SF004181::x&refcode_SF004051=SF004179::x&sort =by_relevance&result-counter=41%20products%20match%20to%20your%20select ion.&page=1

Owain


Owain