It must have been a biggish one at least, to have made it worthwhile.
But a bit daft to forge a big or well-known one, as more the risk to have two of the same in a small city - it would only be a matter of time before someone who has seen one painting will meet someone who has seen the other painting!
It really wasn't a big one. The original is in Jojo's Remembering Robert book under the name Michelle. I don't have the book to hand to upload the image but here is an image of a copy:

The colour is almost entirely different between the two.
Original article:
http://www.thisisplymouth.co.uk/Police-appeal-Lenkiewicz-work-Plymouth-artists/story-15145820-detail/story.html