I've just bought my first buxus and was wondering if there's a good or
bad time to prune them into shape?
Also, are they best left in the pot or put into the ground?

Thanks

Generally box are trimmed/clipped/pruned in June. In this country, traditionally on the same day as the Epsom Derby is run.

Depending on the variety and the size of the pot, they can do quite well in pots.