Even with the really hot summer you have had the grapes do not get enough time to generate the sugars you need. How long are you leaving the wine on the skins as the longer you do so the more tannic the wine will become.

Thank you for that. I leave the grapes on the vine as long as is safe to do so, to catch as much Sun as possible, like for example now, when we have had this heatwave into October. I will pick and start the wine when it cools off (I think tomorrow).

I mash and ferment the whole grape until there is little sugar left. I wonder if the bitterness is coming from the skin or perhaps the seeds (I do not know what the commercial producers do in this respect).

Of course England is probably too far north for good grapes and therefore good wine. The sugar content can be fixed with added sugar but as stated my problem is taste.

I would suggest you do not leave the juice on the skins for more than a couple of days and strain out the pips at the same time. This will help to make the wine far less tannic. The alternative would be to press the grapes and remove the skins and pips immediately thus producing a white wine. How are you pressing the grapes?

I usually press the mash after fermentation, that is with skins and pips still present. However removing pips and skins is a massively tedious job. However, pressing just after picking and then fermenting the juice only might be a good idea. Thank you.

If you are not pressing until after fermentation that explains the very high tannin in your wine.