On 17/04/2019 14:58, GB wrote:
I had a similar problem and eventually got the door to open by lifting it
within its frame. It seemed to progressively allow the rollers to clear
their slots. I really can't remember what the fault had been but I fixed it
prefectly with a new locking strip mechanism for the edge of the door.
About £20 off e-bay.

Success, I did the above, put a spanner on the square spindle, levered
up the door and then thumped the spanner.....door opened.

Still not sure what the issue is but will soon find it first warm day I
get and take the mechanism out, first quick inspenction it appears to be
working ok, with the hooks and bolts functioning.