Mites on cucumbers
This sounds like spider mite. They create webs on the plant and very quickly take over in warm conditions. They reproduce very fast in greenhouses. If you have webbing, it is a bit late for predators as the numbers of spider mite will be too high. If left the spider mites will turn the plants completely yellow and kill them. If the plants are completely covered in webbing, you might be better to remove the plants. If there is only some, then I suggest you purchase some Savona Fatty Acid concentrate from Harrod. This kills the mites on contact but it is important to spray under and over the leaves to get good coverage and spray every 7 days until the problem is solved. Also make sure old plants are removed before October. This will help reduce the spider mite overwintering. They do tend to leave the plants in the autumn and overwinter in the structure of the greenhouse, so it is good to get rid of them before this. Hope this helps.