Any advice on getting rid of red spider mite please or possible red spider mite. I have some white dust on one of my trees and think this is a symptom

Hi Nick, glad to see you posting. If your problem is Spider Mite, you might see some webbing, or the damage that Spider Mites do to the leaves. Spider Mite damage causes a speckling of the surface of leaves at each spot that the mite sucks the juices from the leaf. You can simply wash the tree, being sure to wash both the top and bottom of the leaves, plus the branches and trunk. Generally, it takes a couple good washings to insure that the mites are gone. Or you can spray the tree with a one percent horticultural oil such as UltraFine or Volk Oil. Horticultural oil spray is my recommendation. Or even spray with a liquid soap solution (1 table spoon of soap in a gallon of water). I'm not really a big fan of soap sprays, as generally it takes three or four sprays to achieve control of spider mites when soap solutions are used. - Millet

Well, Millet,
I use soap solutions. I combine my soap with denatured alcohol. I use for one liter water (about a quarter gallon) 30 ml of potash soap and mix with 30 ml of denatured alcohol.
That works fine, because the alcohol helps to overcome some waxy insect shields and helps the soap to do it's job: As wetting agent it deactivates the cohaesion of the water molecules and let them enter the trachea breathing tubes of insects, causing them to drown.

And if you will have full impact, simply add some contact insectizide, like phyrethrum extracts or nicotin to get a very dangerous pesticide.