Reds spider mite infecting my hedges

You can apply red spider mite predators to eat red spider mite. However this is not that practical for a conifer hedge. I would suggest you use a spray with short persistence i.e one that does not leave harmful residues, which kill beneficial insects for weeks after application. There are two products that Harrod Horticultural has, which might be suitable. The first is Insect Killer spray, which contains a natural pyrethrum poison. This kills on contact but does not persist on the plant as it breaks down with exposure to sunlight. The other option is Savona Fatty acid spray, which also kills on contact. Both would need to hit the spider mites to be effective and would need more than one treatment. The biological option is to hang Spider mite Killer sachets in the hedge to release predators. I hope this helps.