Do you know what kind of grape it is, Green70? Different kinds need different pruning techniques. The time is always the same, though: midwinter, to prevent bleeding of sap.

Thank you Tugbrethil for your reply .The grapes are black the vine was bought at a vineyard in the south of England,so I do not know any more about the variety.I will take your advice on the time to prune.Many thanks.

in large country houses the head gardener was expected to maintain the vines in the glass house. the traditional time was Christmas day/ boxing day and they were hard pruned. I used to do mine on Xmas day to get out of doing the washing up. I used to reduce it to about 2 1/2 ft tall. they ripened about now through September. mine was in the greenhouse in east yorks. I had black hamburg,a dessert grape with pips.

It would be better to know the exact variety name, but about 90% of wine grapes prefer spur pruning for good production and quality, and easy vine maintenance. Spur pruning involves choosing a main framework for the vine, usually 2-4 arms spreading horizontally along the trellis. Cut all other branches from the main root off completely, and cut everything coming off the main framework back to 2 buds. Most of the vigorous shoots coming from those buds should show flower clusters about 3-5 leaves out. For maximum fruit size, only allow one flower cluster develop about every 40 cm.
Note that wine grapes are bred for maximum juice production and quality, not necessarily for size. Some varieties have naturally large fruit, and some have naturally small fruit, and some just don't taste right as table grapes. Also, if you are comparing them to table grapes in the store, many of those owe their size to spraying with gibberellic acid when the fruit is just developing.
You should know within a year or two whether the variety you have is suitable for spur or cane pruning: spur pruning will produce few to no fruit.