Thanks @ack. Didn’t realise I was building an old version. Think it’s my fault for pulling from github instead of launchpad.
@billwear, it’s nice of you to say that, but I think I’m definitely making some daft mistakes at the moment. If I had taken the latest code as @ack pointed out, I wouldn’t have been having these issues. 
Think I’m burnt out and need a holiday.
Anyway, the end result is good. I can build a MAAS snap for 2.9.2 fine, and I’ll be able to make changes to use in production.
Thanks all.