The plan so far:
There are several portable shower kits on ebay comprising a 12v pump, shower head, on off button, tubing etc all for about £20. The pump can be placed in a water container full of warm water heated by kettle or pan . Probably need about 10 litres which will probably only need about 2 kettle fulls mixed with cold water. The container can be inside or outside the dandy with the hose routed to the dandy roof bar.
Then a shower tray. I want to come up with a folding shower tray with a curtain attached. The curtain can be suspended from the roof bars and have a velcro entrance to form a waterproof enclosure. The curtain should be attached to the inside of the shower tray. A simple LIDL shower curtain should do this for £3 with some old awning tubing for a frame round the top.
The shower tray should be deep and be able to hold all 10 litres of water so as to form a footbath as well. When done, this can be emptied into another container either by using a dinghy bailer or scoop made from an old plastic container, or a drain pipe leading to a container outside. The tray needs to be about 700mm x 700mm x 150mm high which is 73 litres. This can fit easily in the designer.
The only bit I'm stuck on at the moment is the shower tray arrangement. Maybe an inflatable children's paddling pool but I would like a better solution.