Thanks for the replies.
I did eventually change the bulb but it was a comedy of errors !
Firstly, the new bulb somehow fell out of the holder into the reflector unit (even though I was sure was tight in the holder).
I then had to remove the light unit but on attempting to remove the bulb, it somehow fell through into the base of the unit where I could hear it rattling around but didn't know how to get to it !
I then had to go and buy a new bulb which I successfully fitted but cut my hand at the same time !
Next time I'll just pay £5 for Halfwords to fit it ! 