I also noticed a couple of other things in your pic of the pipework... The feed to the kitchen appears to go out through the wheel arch and the feed (to the bathroom?) is using smaller bore piping (10mm ?)

The copper pipe across underneath is 15mm. the blue plastic water pipe is jubilee clipped onto it. It looks like an electrical cable runs alongside it - assume is single core as it's round - bit hot to investigate today (32C at the moment here). Will go and have a further poke around when it's cooler!

Looking at the back of the wall mounted connection socket, the water comes in in a blue pipe, then goes into a white T piece, from where it goes DOWN into a smooth blue pipe and out underneath the caravan and across to the kitchen, as well us UP into a blue piece of pipe that adapts to a clear piece and goes to feed the washroom tap. The brown wire that crosses the caravan underneath then goes into the washroom cupboard. A blue wire and a brown wire (the same one?) seem to go via the fuse in the cupboard to feed the wall socket (and thus submersible pump).

Looking at the evidence I can see in the photos, ours is a pressurised system - with wall socket ES1001, so doesn't use microswitched taps.

Done it !
Fiddly but finally satisfying.
We now have a fully functioning water system supplied from the exterior Aquaroll
Quite chuffed.
First turn on of the taps was a little nerve racking as all the wires going to and from the relay looked like spaghetti junction but worked perfectly.
So I'm now selling on the original pumps and jerry cans (you can find it in the for sale section of the forum).

Spent a very hot Friday under Eribetta and poking around in the cupboards.
Got very hot and flustered 🥵
Finally sorted out the wiring - I didn't appreciate the odd conventions that have been used for colour coding the wiring but after an hour or so it began to make some sense.
Got the relay into the circuit okay without too much faffing about but cutting the hole in the side of Eribetta was a bit traumatic ... seemed like a vicious assault on my new friend.
But with the hole filled with the new water socket she looks really quite smart.
All seems to be working fine.

Thanks to everyone who has offered advice and encouragement

Well done! Glad all went OK - though Friday wasn't the day I'd have chosen to do it... was 32C here - we live not far from Santon Downham, the hottest official place in the UK on Friday. Not often we hear that...!!

If fiting an external waterpump socket use a magnet to identify where the steel frame is in the wall so you don’t hit it.

Yes, did that but I have completed the install now and all working fine, even the power relay in the circuit.