Looks like root aphids to me. I would have a look through a magnifying glass and you might see them.

yes my first thought is root aphid. they have a white fluffy 'coat' that helps prevent them drying out in dry soil.
squidge a bit and if there are aphids there, there will be a pale beige liquid as a result.

wash the roots thoroughly in weak washing up liquid and then spray with an insecticide. destroy the infected compost. pot the plant up in a soil/gritty compost.

or if it is very recent take it back to where you bought it from and ask for a replacement.

Thanks for all your advice :)

Wow! Touch wood, I've never seen that. Are there any visible above-ground signs apart from plant sickliness?

funnily enough 'creepy jenny' that is against the greenhouse appears to have it. I only noticed today when I moved a tray and exposed a bit of the root. the plant is creeping away nicely so it doesn't seem to have too much of an effect but as they are feeding on nutrients the plant is making then I guess a heavy infestation will have an effect. I also suspect when water is in short supply that will also add to the stress of the plant.

Thank you both … The plant doesn't seem to be affected by it. I dabbed meths on it, as I do with mealy bugs on my cacti, and it disappeared.

yes that would sort them Hywel :o)

Hydrogen peroxide might be better than meths Hywel. It's worth a look?

Thank you, that's something to think about.