I have a Black Hamburgh grape vine in my polytunnel, it is 20ft long and 4 years old, i took 45 bunches of grapes off to leave 30 bunches on the vine, but just like last year all the grapes have just grown to pea size and then started to ripen. :( This year I tried feeding it with tomato feed and giving it plenty of water, but it has made no difference. has anyone got any ideas before I take the bowsaw to it. :'(