Done it (or rather my mechanic neighbour did it) in the end. Sorry for the confusion, i didn't mean i used any form of sealent. It was just when i first put it in i crimped the rubber seal when it popped off the lip. It was a real bugger even for him to do and bloody fiddly. the housing has just two bolts and bloody difficult to work it around the bolt hole for the power steering pump whilst holding the stat and seal in place. after all that the water pump bearing has gone and there's another leak from a pipe. oh well, can't wait to bleed that system again....
