I wonder if it's just got old for the pot/situation. Have you freshened up the compost rather than just feeding? I know vines will live for ages on poor soils, but living for ages in a pot is different, and it may be that there just aren't enough supplies for it in that pot.

Thanks Worthy1 - Well I didn't freshen the soil this year but I did the year before. Do you think it would benefit greatly from new soil every year, and should this be done in Autumn or Spring please?
Thank you for further advice

Are you pruning it well in winter? A correct pruning is very important for good fruit production.

Hmm, I am not an expert, but *think* I have been pruning the way I was shown many years ago, however, it is possible I may, by now, be making mistakes . I clip as near to the main trunk as I can. Perhaps I should try to find more illustrative information? It hadn't occurred to me that this could be the reason for the almost total absence of fruit :-(

There's lots of information on the internet about grape vine pruning but I find a lot of it difficult to understand. All you have to remember is that grapes grow on new growth coming off what was last years growth. In December prune back the previous seasons growth to 2 buds.

Thank you, that is exactly how I feel about a lot of the information. Am I still correct in thinking I cut right back to the main trunk so there is none of the new growth left?

No you need to leave two or three buds on that year's growth, if you cut back to the old wood then you wont get any grapes. Do it late in December or early January when the weather is cold so that the vine doesn't bleed sap. I'd still repot it mind you this autumn.

Come December when the vine is dormant and has turned to hardwood you need to prune the spurs coming off last years growth back to just above the second bud.

In spring when the sap begins to rise these two buds will become the new green shoots for the grapes to grow on. You should also cut away any spurs that are close together right back to the trunk. As a rule of thumb any spurs that are closer together than the width of your fist should be cut away. In deciding which one to cut, you can either leave the one that's growing more upward or if one of them looks stronger and better then leave that one.

Hope this helps?

Well it is obvious I have pruning incorrectly, by cutting right back to the main trunk. When it was regularly pruned by a relative, it yielded as many as 20 plus bunches of grapes annually, so I will try to follow the instructions which you have all kindly offered.

May I please ask yet another question, the top 2 feet of the main trunk has not shown any new growth for the past two years, and I read that in some circumstances it should be cut back, but I have been so confused with multiple illustrations and instructions that I haven't dared to do this. Would it benefit the plant if I removed some of it? At the very end of the bare trunk are a lot of knobbly bits (sorry, don't know correct gardening terms) which in past years would always bear new growth, but nothing has shown for the past 3 years or so.

Thank you once more!

There are many ways to train a grape vine but from what you describe it appears that either by accident or design yours has been trained as a standard. As it's in a pot this is probably more suitable.

I'm not quite sure what you mean by "knobbly bits"? They could be buds, but I think if they were they would have grown into shoots by now. As the top part hasn't produced growth for the past few years I can't see any reason why you can't remove it and this will keep the vine more compact. Do this when you carry out the winter pruning though when the vine is dormant and cut the trunk down to just above the highest bud.

It would be helpful if you could post some pictures so that I have a better idea about what's going on.