Quick update for anyone who is curious.
sirseand kindly passed me a sealed copy of MS DOS 6.22 and the monitor itself
once his replacement arrived, and over the last couple of weeks I finally pulled it out and tested it.
To start with I got this:
I discovered this monitor also goes by the names Hyundai HMM-1200, and Commodore 1402.. which led me to the hsync circuit
I went to an electronics forum and they pointed out that all of the horizontal deflection power goes through C505 and if that was faulty (or anything else in that bit), I'd get that problem.
I went through and checked continuity / behaviour of each component in circuit then started de soldering and testing caps.
Firstly if we look at C505 we see a problem I should have jumped on immediately:
If you compare it's size to the PCB circle - you'll see it's clearly not an original part and likely insufficient for the job at hand. I also matched it's photo to the jaycar website (although they sell a different model now) and various Alibaba/Aliexpress type sites - it's a no datasheet + made in china type deal.
I removed it, and tested it as being 1uF (10% of it's original value).
I replaced it with another NP electrolytic - and tada - perfect display!
Unfortunately I don't have a photo, because I shorted the board by accident afterwards (I'm special sometimes) and took out the fuse. I'll grab a new one on Saturday when I'm out.
Also my replacement cap, is also too small and is not going to last much more than a few minutes at a time - I'm waiting on a proper replacement designed for this application to come over from the US.