[BillyMG]

Having inspected the old one I can see that the O ring on the straight pipe that enters into the thermostat is flat as a pancake and an extremely loose fit. No doubt at all it would have been leaking. I can't see any weakness in the body itself so I'm going to replace the O rings on this one and keep it as a spare. Hopefully adding the clips on the new one will give it some rigidity and lengthen it's life - I presume they go in the grooves on the pipe. I should have time to put it back together again tomorrow - big thanks to Canonite for the 'how to' keyhole method - loads of room to get the job done.