I've only just found this thread, so I will add my pennyworth somewhat late.

Greenhouse vines are typically grown on a single 'rod' - this is the main stem and can be quite long. Once the rod has grown to the limit of your GH, winter pruning when there is no sap around (this is the bit that everybody gets worked up about 'bleeding'), cuts out the old hard wood such that every 9" on the rod there is a short spur with 2 buds on it - short being in the order of 2".

Once these buds have started to grow, you can nip out any of this year's shoots, but not any of the hard wood from previous years. The aim is to have one side shoot from the rod every 9" or so, with the side shoot being around 18" long and carrying one large bunch of grapes.

If you don't want large bunches, then my mother's 'just-tidy-up' prune and let everything grow willy-nilly approach seems to work as well but the bunches are far smaller, but more numerous. As my mother is 88, this approach suits her better!

Rob