I was told by Citroen and the supplier of the replacement thermo I did NOT need the additional wiring harness as sensor cable would reach. Well incorrect boy wonder....luckily I had a spare wiring loom and cut and connected an extension. I would assume all early VIN cars need the extension cable??Leaky water boxes on early EP engines were a common warranty item, (as were leaky pumps). Most are probably fixed. Look at where the large pipe joins the body for failure. With the integrated thermostat you are up for a new part. You may get P0117 or P0118.
The reference earlier isn't an official site, but a user who seems to have dismantled everything.
For the pump, you work from the wheel arch. I assume you know how to release and remove the friction wheel. If you have cables around the pump set them aside as a unit.