I think I've sussed it now...
It didn't occur to me before but I now see that with a lock using a curtain it is the curtain that throws the bolt, so the tension part of the pick is only long enough to move the curtain and wouldnt be long enough to throw a bolt without a curtain.
Looks like I need a set of overlifting picks too then!

The list goes on and on...
Thanks for the help lockey. I did mean the curtain pick and not the 2 in 1 pick- the 2 in 1 looks just the same as the Overlifting Pick but it manipulates only one lever at a time I believe?

For the overlifting picks, is there a common gauge? or gauges?
I notice in the catalogues I have that some manufacturers only make 6 & 7 gauge 2 in 1 and overlifting picks - but curtain picks generally come with 5 & 7 gauge bodies???
I'm just wondering which gauge of overlifter to buy first?