Hi Paddlers, yes managed to get it off the downpipe, took my time so as not to damage it and finally came off - I'll be sure and grease it for ease of removal next time.
It should have been inside the top of the impeller housing, bit strange why it wasn't, but I'm guessing that would explain the lack of water flow since it wouldn't have been sealed properly.
I'll fit a thermostat, will make it harder to check if the impeller is still working, but as you say it should have one so that's what I'll do.
Just checking prices here, those little stats are pretty expensive are they not !?