XF rear calliper pot spins but won't screw in

[Stunned Monkey]

As per subject - it moved in at least 5mm but then stopped moving any further and just spins - quite freely actually. I've read a couple of people suggesting undoing the bleed screw. I'm not sure how that will help but mine's now beautifully rounded off (with a brake pipe spanner before you ask!)

Also got a G clamp on there just to check and sure enough it won't move.

Any advice?

[Stunned Monkey]

Update: Managed to loosen the bleed screw with mole grips, then applying a G clamp gave a squirt of fluid and the pot moved, but not all the way.

The opposite side did the same thing - spun, moved a bit, but wouldn't go in.

I'm NOT using one of those special tools, instead I use a pin spanner. This means I'm not applying pressure to the pot - don't know if this would make a difference but hasn't been a problem on all the other non-Jag callipers I've ever done