well I have gone for it ,many thanks to Sub Driver for his help. attached some before and after pictures so far,
I have opted to do away with the mechanical switch for a magnetic one from Engel , I have used the spigot where the switch pull rod came out for my water pipe entry/exit, the tank I have used is an 825 TA with the BTS2 switch,
one small problem that I encountered was the valve in the hull for pressure testing protruded too far into the W/T cylinder that it fouled the Engle tank and also the fact that the boat was already built meant that I had to cut a section off the lower after casing to get to the rear bulkhead fittings ,I cut it about 130mm back from where it fit to the main hull and added doublers to the join to stick it back, Service from Engel is first class I ordered the tank on line on Last Friday afternoon and the tank was delivered at 8.15 Monday morning, the magnetic switch I ordered Sun Afternoon and got it this morning!!!

more to follow later.