Bugger.
It turns out that the AP Racing master that I've got isn't really designed to be user serviceable, and I'm unable to get it to bits. Speaking with AP, they said they made the kit available as some customers were making their own tools for stripping them down, and that they otherwise re-kit them at the factory. That'd cost £75 not including the cost of the seal kit - unless other parts need replacing - and I've been told it is one of the first of this model master cylinder from the 90s so it's fairly old now. I was keen to give it a go but they're unable to give me a time to send it in due to the current circumstances.
So I'm going to replace it.
Having had a look at ebay for second hand original master cylinders, there doesn't appear to be anything local vs the states, and tbh I think I'd rather put a new part on anyway. So I'm considering this:
https://www.ebay.co.uk/itm/193037768382.
This does possibly delay the tyre replacement :/