Hi Mark,
Welcome to Permies!
Red spider mite are pretty persistent in my
experience. They like hot dry conditions, but seem to overwinter successfully too. I found
this site which gives some details on the spidermite identification and lifecycle along with some predatory mite information. It is possible to buy predatory mites by post.
It may be that once the plants have a chance to settle in the mites will rebalance naturally, I used to have problems in my polytunnel, but after introducing predatory mites and a
pond and using mulch and trickle
irrigation to increase the humidity I don't notice the mites now after a few years. I think they are probably still there, but with better balance.
It might also be worth contacting the supplier of the plants for advice.