Hi Paul,
I've got an S40 (1988) and my cable snapped last week!
With the odd friendly expletive, I got in through the main front grille with the volvo sign on. It took a bit of prising with a couple of flat screwdrivers but it came away (I ended up snapping a couple of the bits of the grille but they'll glue back together nicely).
I undid the bonnet catch bolts. There are three of them and they are 10mm.
Once these are out the catch should either come up with the bonnet (depending on how siezed your catch is) or you'll be able to open the bonnet enough to unhook the snapped cable, so you can open the bonnet fully. The cable is attached to the catch on the left corner (as you look at the front of the car)
It was quite an easy fix...I ordered a new catch (as mine was completely siezed) off ebay - the shop's called 'classicvolvoparts'.
I hope this helps! All the best!
Pete
Pete Fryatt, May 2008