I can't recall there being anything tricky or complicated (couldn't be could it, 'coz Halfords can change bulbs).
Could be the rubber seals are sticking to the bodywork/paint.

 

Your doing it right , undo the 3 knurled plastic knobs & pull light unit out . It probably needs a wiggle first to get it loose before you pull .

 

Thanks guys. It was te rubber grommet & stud that was sticking. After a good wiggle, it came off. Thanks again.

 

Understand your problem I had broken glass and watched you tube instruction which were totally misleading Scared of doing damage I spent time to research eventually found trapdoor and undid screws. It had to hammer top of screw but carful not to damage tread and wriggling light a lot. Now have new light fitted so can sell the back unit with fully functional lights (glass is broken) pt no is 39125831 £75 including Del