They can overwinter on the plant not the soil at any stage but overwintering eggs are a sure shot for causing trouble next season. It's the armored females that have to be done in though they can reproduce without mating( parthenogenesis ). Be vigilant next season and any sign of trouble you can use what's is available to you in the UK for control, an oil spray perhaps?

So does anybody in the UK know what I should use please? Are the eggs visible and if so how are they recognised? I am cutting off all the affected parts i can find - most of them are beyond help but so far it looks as if only one smallish area of the bush is affected.. There are more than I thought there were, once I kneel down and look underneath. Do they overwinter on the leaves or the wood? And do I need to spray upwards to get the undersides of the leaves where they seem to have been? I think a systemic would run off the shiny leaves.

In general the eggs usually overwinter under the mother scale and this can be either on leaf or wood.

Very useful info Loostrife. I will go out and hunt them down - all the ones on the dead branches are now on the bonfire pile but I'll check for more.