OP
Octopus
UPDATE:
Sunvic Technical : 01698 810945 (who were excellent)
Have confirmed that the valve in place isn't correct (I think I knew that) and that it needs to be swapped for the SZ 2301 (valve normally closed, switch normally open). The body is the same so just a "simple" case of swapping the head.
Happy days!
UPDATE:
2 "happy" texts today :
1st from customer
2nd from plumber (he fitted the replacement head last Friday).
A least this is now well and truly sorted!