Can anyone tell me what the best time is to plant apple trees in the UK? These trees will be on a trellis and will be spread wide not high.
Doesn't matter whether you're fan training, espalier or cordon growing, the best time for planting trees in the UK is September or October. If they're container grown plants, you can plant at other times, like February or March, if the ground is neither waterlogged nor frozen, but you will need to pay careful attention to watering during the first and second year, especially the first summer.
As requested, I'll expand the answer. In the UK, the soil is still warm up to end of October, but the heat of summer is well past and we're usually pretty wet during autumn, which is why autumn planting is recommended for long term, woody specimens such as trees and shrubs. The weather here supplies sufficient moisture during autumn and winter, and because the soil is still warm, roots settle in quicker. Bare root trees and shrubs are best planted October, not September, through to early November at the latest, for similar reasons, though the timing varies somewhat depending on which part of the UK - in the south, planting is best in October, in the North, September.