Thanks for the advice. I would definitely be upset if it spread to the other plants. I think I'll go with isolating them for the time being (in another room) then think again when it gets to final potting up time when they couldn't be isolated any more. I kind of want to know whether they will fruit, now.
Could it be a mutation sort of like mutant candlelight (although not as pretty)?
Thanks for all the suggestions. Yes there is some algae; it formed on the peat Jiffy 7 pellets which were probably kept a bit too damp. When I potted them up into the Biobizz Light Mix it hitched a ride. However, this is the case with all of my seedlings and not just these particular ones. I definitely haven't overwatered since potting them up, they have had very little water which has drained through and been emptied out, then the compost left to dry out between waterings.
All my seedlings have been treated exactly the same and all the rest of them look healthy and "normal". They are in the same compost, watered with the same water, under the same lights, everything exactly the same. The only plants that are like this are the Brazilian Ghosts which makes me think the cause is seed-borne and not environmental. No one in the house smokes. The suggestion of a tobacco mosaic type virus is the kind of thing I am worried about because I don't want my other plants to "catch" something from these, if it is a virus. I have a few aphids on the plants now, but there weren't any when the deformed leaves started to form; the plants were free of pests at that stage.
The seeds came from the European Seed Train on this forum but there is no username against them so I don't know who put them in the train, otherwise I would ask them.