Did the overwatering wash them out? If so, ideal, flush 'em all out, and re-pot! Don't ask me what they might be. In fact, I'd hate to think. Sounds quite a selection! Seriously, if it is an infestation, then perhaps the way forward is to wash all the compost off the roots, and very carefully re-pot in new compost. Worthy

Cheers, It was like something from a horror film! Just hope my other indoor plants are ok? Will redo the lot I think.

Use a gritty an fast-draining compost, too, Bexx. It may be worthwhile sprinkling some organic mosquito granules, or a broken up mosquito ring into the compost before repotting, too. That contains a kind of bacterium that kills larvae of the fly family, such as fungus gnat or cranefly larvae.

Will do that thanks. Do you think my other plants will need the same treatment? I've got lots of tiny black flys that seem to be around them.

Those are the fungus gnats, so I would apply the mosquito granules to the soil surface and water in, and try to water less often--they are frequently a symptom of overwatering. At the same time, pour chlorine bleach down the drains, because they will often colonize the drains, too.