I thought I should let you know that I finally succeeded in fitting the new retractor.
The unit is essentially wedge-shaped (side profile) , and fits very snugly (tightly?) between its metal shelf and the sloping aluminium bodywork above.
The replacement unit, while very similar to the old one, is just a few mm higher, as a result of which (in my 2001 +8 anyway) it is displaced towards the front of the car slightly, and the retaining bolt-holes in the shelf and unit do not exactly coincide.
I resolved this by shaving down the top of each metal side of the retractor with an angle-grinder, and extending its bolt hole with a Dremel.
Inserting the new bolt involves using a pair of forceps, as it is taller than the space above, and has to be offered up at an angle..
ps I'm happy to insert photographs, but seem unable to find how to. Can anyone advise? Thanks