All seemed to be going well, it slowly over 4 days worked away and got to 1.020 and then got stuck. After 2 days stuck at 1.020 I gave the fermenter a good 'swishing' to try and rouse it a little but that doesn't seem to have helped any
So, small update - mostly I wanted to post this here to help anyone else struggling with this yeast, when I researched this I found a LOT of forum posts here and elsewhere with people reporting MJ M20 getting stuck at 1.020, and in only 1 case did the poster say they left it for a few weeks and it came back and finished.
Well, after 4 days fermenting down from 1.050 to 1.020 and then 16 days stuck at 1.020, it finally went back into action. 2 days ago it was still 1.020, today it's 1.016 - and such a small gravity change has made a huge difference in the flavour, before it was noticably too sweet, now it's nice and dry.
Slightly annoyingly, I measured it because I just took delivery of a pack of fermentis Safale WB-06 that I ordered 2 days ago. When it was stuck for 2 whole weeks I decided to give up on it and pitch a new yeast to try and finish it - new yeast arrives and I find the M20 decided to start working again. I guess I scared it back into action heh.
I'm hoping it'll continue down to 1.014, to hit my target ABV.