im guessing you replace the bulbs from the inside.
if its a rear light, there will be a cover of some sort on the light. once this is removed the bulbs usually snap out on a circuit board.
with front lights its often done from inside the bonnet. just a snap or twist fitting in most cases..
:-)
I may be able to offer some assistance if I knew what year an "n" reg was - they dont go by letters over here ( N.I. ) and which lamp needs replaced - front or back ?