Thanks for the reply.
I found some spider mites on it the other day, but Im sure the colouration predates the spider mites (as I check them regularly). I have started spraying the leaves with water to try and get rid of them.
I used to feed them BabyBio but when I read on the internet that too much Nitrogen can harm them I stopped feeding them (this was around 2 weeks ago). I do think that I could have over-fed them previously because I tend to be a bit "generous" with my plant food.
I try not to water too much, maybe 1-2 a week when it is hot.
Is it possible that they have tobacco mosaic virus?
Do you think it could be spider mites (that I may have missed on pervious inspections) causing the problem?