Thanks guys, appreciate the comments.
I had a flash of inspiration today and remembered a physic lesson about applying force through levers. I put the end pin back in and with it tightened in an extended position was able to apply a suprisingly little amount of force to wiggle the collar free!!
Have ordered a new end pin unit which should hopefully arrive on Wednesday so fingers crossed everything should be sorted in time for next weeks gig.
As an aside, the bass is currently flat on its back on the spare bed, much to the consternation of the wife. I've explained about the risk of dropping the soundpost in its current state but she wasn't impressed as her mother is due to stay at the weekend. A few choice words were hear when I suggested there would be enough room for the mother in law to share the bed with it :-)