It's not that difficult...unless like mine the longer of the 2 pins that hold the trigger unit to the block won't come out, I used the correct punch, tapped lightly (as advised by a youtube clip) hit it harder...and harder...where's the club hammer, ok not quite that bad, eventually out it came, on fitting the catch would it go back in...would it. **** the hollow pin it fits through wasn't really big enough! I had to run a drill bit through it.
All OK now, if you don't have the problem I had (I'm lucky like that) it's a piece of cake.
Looks good now fitted. Just need Rowan to get their finger out and make some more brass set back triggers.