Lets hope not but I don't think its wise you attempt a DIY job because it may become more serious.

Barry has just tried to move it ,and it has now totally jammed,so looks like a repair job after the New Year.Do hope it can be repaired.

It can be repaired I have seen this happen a few times.

Some times the locking collar gets stuck, try holding the front of the lens and roataing the collar counter clockwise. Also some times the lens barrel gets gewy, best way to clean is with alcohol, put it on the cloth not the lens and wipe, it desolves right away, but do not get it on the elements.

Thanks,Phil for advice ,but I guess I will give the repair people a ring after the New Year,and ask their advice,see how much it will cost etc.I have been using this lens for 2yrs so I guess it does perhaps need a service .