Thanks for all the responses.
Unfortunately it is still stuck. I have tried all the suggestions (got an oil filter wrench from a friend) but with no success.
The meat I was mincing was not frozen and had no skin on. I'm not sure if there is something wrong with the alignment of the screw, because even though it appears to be assembled correctly, it will not turn at all.
Anyway, I am taking it to Franco this week who kindly said he would sort it out for me......fingers crossed