Looks like we've got a new member of the Aquatic Collective!
It's not so much of a mystery creature, as I think it's a Lymnaea stagnalis The mystery is more how I didn't spot it earlier! I bought some hornwort on eBay a few weeks ago and gave it a really good going over before it went in the tanks as there were a few 'critters' on it. Then, while cleaning Howie's tank last week I moved the filter intake to one side and lurking behind it was this fella! It's about 2 inches long! I'd never seen hide nor hair (or even shell nor feelers!) of it until then, obviously been lying low. It was out and about this morning though, climbing up some hornwort.