Hi Thanks to everyone who replied to my last message. It was very helpful, and it gave me the confidence to have another look.
As suggested I probed around the M33 chip and everything was where it should be except the link between pin 4 of M33 and pin 20 of the UART. So I took my courage in both hands and soldered a bodge wire to connect the two together, which was actually fairly easy as they are in pretty much a straight line, and whilst M33 is surface mounted it is on the back of the PCB. I wasn't hugely confident... but I reassembled everything and to my great delight I found that everything is now working properly and I am receiving serial data as I should! So a big thank you to everyone who replied, although I was fairly sure I don't think I'd have had the confidence to try otherwise. I'm not quite out of the woods yet as have a slightly odd keyboard problem but I'll do a separate post about that. James
