They're aerial or adventitious roots - common on greenhouse grown vines. No one's 100% sure why they form, but it seems to be a response to either a shortage of water, or a response to cold.

The leaves on the vine look a bit discoloured too. Is the root outside the green house or inside? Maybe there is a continuing lack of nutrient and water if the root is inside. As suggested by Bamboo, this is the response to that particular environment.

Yes, the roots are inside, and the soil inside is quite tired as I set up the greenhouse about 15yrs ago. On this advice I will think about replacing the vine and putting the roots outside the greenhouse.

Thanks for the comments!

Tom

It might just be because of the much colder than it should be weather during what's laughingly known as 'summer' this year - is it possible you can improve the soil round the base by adding composted animal manure or something similar? Also, if you forgot to water sufficiently (we all do when the weather's bad) that might explain it.