I have woolly aphis on my apple trees. Can it be treated and when or should I dig them up and start again?

Don't dig them up! Go to the garden centre and look for a spray which will treat the woolly aphid, though make sure its okay to use it on food plants if you want to eat the apples.
You may have trouble controlling it this year, as its now late summer - next year, keep a check on the trees from Spring onwards and treat at the very first sign of it.