The verdict is that these are actually spawning brook lamprey, here’s the reply I got:
I would most likely go for brook lamprey (I’m quite confident), as it appears they are spawning. The sediment also matches the type they prefer to spawn on. Their also appears to be several gill holes on one of them in the third picture although it is hard to tell. It is a shame as after these spawn they will all die but it must have been fantastic to see.
That’s really good news! Spawning Lamprey on the canal. I don’t believe they have ever been recorded on the canal and from very few locations in Swansea before.’
Rhiannon is forwarding it on to the appropriate recorders.