Went at it Ferret style!!! Ended up being the 'float' type connection on the intensive clean bottle I think. I removed both tanks and all the connections and cleaned the contacts on the floaty one ( Leicesterboy!) - cleaned the crap outta the tanks and put all of the connections back in making sure float was correctly orientated.

No more annoying gong... THATS what I'm talking about!

Many thanks to all - A cheap (free!) easy fix for once... I like it that way.