So, it looks as if no one knows. I think the master cylinder leak is a first on TM?
Depending on where the leak is, I would suggest a couple of options, though I'm sure you have already pursued these. No 1. is if it is a seal, then go into your local motor factors and try and match it, if it is the plastic resevoir, it will need replacing, and I suspect that any resevoir with a similar size would work, it's just a matter of finding one of the right size, again your local garage maybe? 3rd, email Morgan direct and get them to express freight one out to you.
Best of luck with it, keep us posted.
And..... oh yes, welcome to the forums Alan, I hope we'll be hearing more of your 'adventures', maybe enhanced with some of those great sky films of the Three Wheeler.