On Saturday, 24 February 2018 18:09:13 UTC, Bazza wrote:
Unable to open aluminium patio door as upper locking point will not release. The key and lock barrel work OK. Both the lower locking point (hook type) and central mortice lock release OK.

The lock is model L22180. There was obviously a problem with this type as Union ( now operating as Yale) no longer make the connecting rods and the locking bolts.

Short of drilling several holes into the door frame and cutting out a small window to get access, has anyone got any suggestions on how to release the upper hook?

Try carefully prising with a jemmy.
If it's only one lock out of three you might get lucky.