It's interesting what stepping back from a problem, and then returning later does.
I have answered my own question.
The solution is to rotate the inner sprocket carrier until the bolt lugs protrude beyond the sides of the front case.
Place timbers under the lugs so that the case is clear of the ground, then gently tap the centre with a suitable socket
until the case drops free. This keeps the bore always central during movement, and so preventing it from binding.

I hope someone else might find this useful, I don't expect this task is performed very often as rear sprocket wear
is minimal, thanks to the fully enclosed nature of  the assembly. The rear sprocket on mine has never been changed.
where as the front sprocket is soon to be on the 3rd one due to spline backlash not tooth wear.

The chain is also the 3rd  fitted, and is, 630 Heavy Duty Non "O" Ring,
when laid out doubled up, all pins align exactly, no uneven stretch.

In the past I fitted an "O" ring chain of the correct pitch but this didn't last too well, and O rings are not recommended.