"One thing I'm struggling with is getting the indent spring holder out of the case (it's got a step worn on the back edge). Am I right in thinking it's a push fit and knocks down and through the bottom of the case, I'd hate to think I was trying to force it the wrong way or that it was threaded!"
Good question Guy, but, no, it actually knocks up through the case from the bottom (outside). Although it isn't threaded it does have a step (see photo), which prevents it being knocked through the bottom of the case. Perhaps the accompanying pictures will help.
Anyhow, knock it out from underneath, so that it 'pops' out as you see it here. But, as for fitting a new one....
I found it best to warm the crankcase half, having left the new holder in the freezer overnight! Trouble is, it soon warms up when it come into contact with the warm alloy, so try to be quick, whilst, at the same time, keeping the holder aligned at the correct angle.
Finally, it's not easy to tap it place from inside the case, but that's what has to happen, as the mounting point for the selector bracket you can see to the right of the holder gets in the way. Best of luck. It can be done - I've done it - and it'll make all the difference to gear selection with a new holder in place.
Thanks for your interest in my website, Guy!