Excellent advice so purchased a new Gib strip to understand exactly what I have.
Success, as advised thin end at left hand side when facing the machine and following a few taps on the thin end all was released table and gib strip with about 8 mm broken off one corner edge at the thick end hence the jamming.
How or when this happened I have no idea
If it helps anyone else in the future with this problem the gib strip dimensions are as follows
Length 175 mm
Thickness and height at thin end 4.85 mm & 15.87 mm
Thickness and height at thick end 6.30 mm & 16.30 mm
This episode much reminds me of my late father's advice that "time spent in reconnaissance is seldom wasted"
Many thanks to all for your advice