Hi, sounds like glasshouse red spider mite, if you don't mind using chemicals you can spray with ? well I was going to say liquid derris or malathion, but I think both of these have now been banned, check at garden centre, if you don't want to use chemicals, they can often be controlled by introducing a predatory mite called Phytoseiulus persimilis, infestations are often found where plants are grown in dry overcrowded conditions, and can be reduced by frequent syringing with water, including the undersides of leaves, Derek.

Thanks Derek. We'll have a go with the syringing..

I wouldn't use any chemical insecticides on anything you're going to eat. Spray with a soapy water solution, washing-up liquid will do fine.

I used some predators which I bought online from Ladybird something or other. They are supposed to work but by the time they came my cucumber plants had expired.

Thanks also to myron & cammomile, we'd tried soapy water but to no avail :(

Sometimes you need to give it 2, three or even more applications of soapy water, make sure that you give the leaves a good soaking, especially underneath and wait a couple of days before doing it again. An organic pesticide that I find works for stubborn aphids is a mixture of 2 tablespoons of washing-up liquid and 1 tablespoon of cooking oil dissolved in a gallon of warm water and shake it all up to dissolve. Yes! The oil will dissolve because the soap acts as an emulsifier. The only problem with this is that you get an oily residue on the plants, but this seems to get rid of the blighters. I suppose it depends on what you prefer on your plants .... Oil or Aphids?

Definitely oil, me thinks.. I'll give it a go & let you know.