Back from the dead! Sorry about this long absence and particularly apologies to Totally Not Ben whose contribution I did not acknowledge.
However, I have found the solution to my own problem which may be of use to anyone encountering the same problem. The thing is that the Blick 5 does not have a knob on the left end of the platen as does the No: 7. Logically, I thought, the left hand knob would have a left hand thread, however, this is not the case. In fact both knobs have a conventional right hand thread.
You may have noticed that a six month delay has occurred since I opened this thread. It took this time for me to raise enough courage to try a little force, there again I have 83 years of life behind me and things move fairly slowly at this age!
Happy tapping.