No shop (and petrol station, DIY market ...) around here had this wiper in stock. Eventually I bought it from a reputable ebay seller. Do a search for Bosch H351 and you should find what you're looking for.
BTW there is BOSCH printed in big white letters on the replacement wiper. This word will be visible upside down on our cars. But you can replace just the rubber piece.
I'd probably say one of the best paces to look is breakers yards where you'll be able to get one in fairly good nick - maybe even with a rear screen still attached
So the rubber surround is nothing more than a trim, as the rear window is bonded in. Should be easy enough to get one from a breakers, it just unclips.
If the problem is a leak, replacing the trim won't help as it doesn't provide the seal.
Thanks for your help, I have been told by a fellow MG enthusiast that there is a good breakers yard not too far from me in Bolton so I will look there.