I'm no expert but I'd guess they are detritus worms, which aren't really harmful. Just need to be careful not to feed too much, and vac the substrate/clean filters better.
Agreed, they don’t look like planaria. But they’re still a sign of heavy organic load and possibly overfeeding or under cleaning so be aware of that.
Thanks for the replies.
I do a 30% water change weekly and clean the entire filter out each time. I’ve not really got much options for gravel cleaning, because it’s a heavily planted tank (around 80%, just not quite reached the tank edges yet). Are there any other ways to get rid of them?