Ok so just to re-cap on how it went in case anyone else has this issue.
So I went to the barn last night planning to work on something else as I'd hit a dead-end with the jack but I had half an idea that I'd try pulling the remainder of the 1st grub-screw out with an extractor - just to really make a See-You-Next-Tuesday of the situation
Luckily I had none of a suitable size and then for some reason decided to try and rotate the castor by hitting it with a hammer (not too hard) and I was pleased to se it moving slightly. After a bit of to and fro with this I noticed a metal bit in the groove of the pin.
Turns out the end of the screw somehow came loose and fell inside. I turned the thing upside down and the little bit fell out so I continued with the back and forth using a big tyre-lever and eventually the castor came out.
The design was similar to what I'd imagined except I didn't consider the end of the screw would be unthreaded and of a reduced diameter. Also didn't bank of the 'pin' being so wide.
So thinking back to when I was drilling I remember a void appearing at the bottom of the screw and assumed I'd drilled squint or the casting was porous but obviously I'd just drilled into the groove on the pin.
I still think I got lucky that the end was weakened enough to fall out but you can see from the pics that I drilled way past what I needed to.
Another strange thing is that altho my drill hole was reasonably well-centred I didn't hit the mddle of the groove on the pin.
Putting the castor back in shows that with the groove in line with the screw-hole there is a considerable gap at the bottom. Perhaps big enough to have a bearing-race in....
With the castor pushed right home the groove is partly above the hole. Bit strange.
So after getting the first one out I felt that since the screw wasn't very long I could perhaps pull it out with the help of my MIG, rather than drilling and perhaps not being so lucky 2nd time. I'm still not sure how I ended up with nothing of the threaded part stuck in its bore.
So I tried welding a nut on. This never works for me. Despite lots of heat and weld the nut just tore off and I didn't have any other spare ones around so I just welded a blob on the screw then worked it with a pair of vise-grips. As with the first one it turned easily at first then seemed to stick so I feared this one had snapped somewhere too but no it came out complete.
After that it was lots of to and fro with large bar wedged in the castor and lots of ATF drizzled in but even with lots of up and down movement it wouldn't come our for ages but lots of cycles of twisting, dousing, knocking up then down eventually released it.
Lastly. You will see from the pics there is a small pin or something sticking out the bottom of the castor pin where it's welded to the 'saddle'. When I was working the 2nd one free I swear this was stationary while the rest of the assy moved.
I was thinking perhaps the main pin is not one piece. I can't see any signs of a collar or anything and it seemed solid once it was out but I could swear the thing moved independently of the skinny pin. Why is that there anyway?
Oh and lastly I also got the large ring from the ram undone. I thought this would be a major issue and was thinking of making a toothed tool out of steel plate to loosen it but with some deft blows on varying teeth (to avoid mullering one) it came free. I thought the ram would come straight out of the housing after that but I guess whatever seal is inside is still gripping it tightly so, as Edd China says, that's a job for another day
1st screw and a little bit that fell out the end woop woop it's out I drilled into the main pin. assumed it would be narrower with castor pushed fully home (note drill hole) gap between castor and frame looks normal with groove on pin centred to screw-hole gap between castor and frame looks too big to be empty nut welded to 2nd screw was not a success so welded blob on top instead and got screw out. note reduced end large bar used to get things moving even with lots of up and down movement took a while to get the castor out large threaded-ring on ram fully loosened