OK, so I went ahead and did the bottom catch replacement today, didn't go as I thought, but nothing ever does!
So unclipped the bottom half without too many issues. Take note of the spring locations as you do this.
From here, I could see the screws deep in the top, but didn't have a long enough driver bit to get to them. All my extension bars had been too fat, with the screw heads next to the mini cyclones, it just wasn't going to work.
So then remembered you did it without taking the top off. So had a bit of a look, and got the old catch out.
For those playing along at home, split the cyclone, then remove the spring on the catch, the catch will then with a bit of gentle force 'pop' out sideways. Put the new one in, by threading the top through the hole and then gently pushing the bottom in. Again it should 'pop' in sideways without much trouble.
Make sure the spring is installed and catching at the top, as its needed to 'push' the catch out.
The HARDEST part was yep, getting the springs all lined up. Its a three handed job. I got an extra set of hands to hold the catch spring with half a clockwise turn on it needed to push out the catch. They then held this with some pliers.
I then got the grey catch spring in location, once all lined up. clipped it back together, removing the pliers at the last min so the spring still had the twist tension on it.
All good the bottom catch has tension, so it is pushed away from the unit so it lines up with the bottom bin parts, the grey catch is working. No broken plastic bits etc. So win win!
Now would I remember this method if you have the older catch like I do and needed to replace both parts? Maybe not. For me in Australia, it cost $20AUD for the catch shipped, but I did notice Dyson sells the cyclone for $80AUD shipped. So unless you're great with pulling these things apart, for the Aussies just buy a new cyclone.
I did notice in the UK the price is a bit different, $5 vs $55 pounds, so clearly YMMV and of course I'm sure if you dropped into the lovely people at Manchester Vacs, they would do it all for you!
$5 pound option - https://shop.manchestervacs.co.uk/dyson-upright-vacuum-cleaner-spares/dc41-spare-parts/dc41-cyclone-release-clips
$55 pound option - http://www.dyson.co.uk/spares/c/cyclone-assembly/cyclone-assembly-dc41-satin-blue.aspx?pid=DC41-Animal
If you do the go it yourself route, maybe think about one of these https://shop.manchestervacs.co.uk/dyson-cylinder-vacuum-cleaner-spares/dc26-spare-parts/dyson-type-t15-screwdriver
Again, thanks for the advice and thanks for supply the part MVacs. All happy now, that the old DC41 is back to working again!