Ok, I have an update and a resolution.
I took my bike to a Dr Bike session, bloke looked at it, had a fiddle and declared that the end of the clip had worn away (or snapped off).
I ordered a new clip online and when it arrived I offered it up to the old clip which I had removed from my bike - they were identical.
So, I backed off the suspension block retaining bolt and realised that it had shifted within the hole drilled in the rear frame. I tightened the nut having centralised it in the hole and, hey presto, problem solved. The frame now locks correctly to the clip.
Lesson learned I suppose, but thanks for everyone's input.