Update, sort of.
A mate gave me a load of followers, I went to change the broken one yesterday, found out I needed a tool to hold the valve down, as that's the only way without removing the head. None in stock locally.
Ordered one, picked it up this afternoon.
Pinned the valve down with it, managed after much poking to liberate the other half of he broken follower.
Could not get the new one in, definitely not enough room.
I'm thinking maybe the camshaft hadn't pushed the valve down enough as the follower was broken.
Tried screwing the tool down more, slipped off.
Got the tool back on, started again, got to what I think is almost there, and tool started slipping again. Was getting dark so I've packed up and gone home.
Might go out later with a decent torch, but pretty pissed off, so might not.
I just wanted to change the broken follower, see how it runs, maybe do a compression test.
Sounds so simple.