Thanks to all for the tips. I finally got it done tonight.
The hot water worked perfectly on one end but no joy on the other.
I couldn't try any of the tips about the string or cable ties around the inner collars as I couldn't expose the inner collars at all.
I was getting really p***ed off until I was sent a handy tip which worked perfectly. Wrap some thin string very tightly in between the outer collar and the endpiece (collar that screws onto the mouthpiece/t-piece).
Although it won't wrap around the inner collars, eventually (after about two meters of the stuff!!) it will fill the gap and jam the inner collars against the end piece. Then the outer collar will release from the inner pieces.
Again many thanks to all who took the trouble to reply.
Mick