Too late in the season. They need warmth. (and you won't be able to use them after you've used insecticide)

They can be quite fiddly - you can't use them until the greenhouse/conservatory is warm enough and by then the red spider mite may have built up to more than they can handle - or to more than the number of phytoselius you can afford can handle ;-)

But at this stage of the season, in a greenhouse, the problem is going away - the red spider mite are breeding much more slowly. Not necessarily the case if your conservatory is kept warm all the year round, in which case you may do well with Phytoselius.

How much have you got? If at very low levels, removing the worst affected leaves and washing others with soapy water can go a long way.