I think you need to summer prune, and then thin out the grapes, maybe google it, you may find a demo on youtube

Hi, ideally, grape vines should be planted outside the greenhouse, and just the vines inside, not because they need the protection, but because our summers are not usually sunny enough, or long enough to ripen the grapes, having them inside helps with this, but they are hardy plants, Derek.

I have a grapevine in my greenhouse that presents me with the same problem and every year I give it a summer pruning. I cut back the new growth when it gets to about 2 or 3 feet in length.

Overgrown grape vines often produce much less fruit than those that are properly pruned, so summer pruning will direct the sugars produced by photosynthesis into fruit production rather than excess foliage growth.

I also remove a lot of the larger leaves around the bunches of grapes, especially those leaves that are touching the bunches. This allows more sunlight and air circulation to get to the fruit which helps them ripen and gives them a better colour.

OK many thanks ..
Just googled summer pruning
Thanks
george