I went through the fun of this repair last week. Reggit's thread had some useful pointers, so thank you for that. I'd say it was probably a 6-7 hour job for me, split over a few days for sanity. It through up a few challenges which I'll detail below. On the plus side, although access is horrible it is all from the top, so at least you're not rolling around under the car.
I had the same issue with a rusty bolt under the battery tray...
Drilled out...
...and replaced with a stainless bolt...
One of the bolts holding the EGR exhaust to the EGR sheared off. I tried encouraging what was left out with heat, mole grips, manual impact drivers and left handed drill bits but it didn't want to shift.
I then remembered that I own a MIG welder, and welded a nut onto what was left of the bolt stud...
In a decision that I might regret I opted for an aftermarket thermostat. The aftermarket part was £45 delivered next day from Steve at Trust Car Parts (formerly known as Braydons). It's made by Pro Parts Sweden who have a bit of a mixed reputation for their Volvo and Saab parts, but Steve assured me that he'd sold quite a few of them without any reports of failure.
For anyone else doing this, the Volvo part number is 31368373, the Pro Parts number is 26438373.
It doesn't come with the somewhat flexible pipe that runs into the EGR. I had a similar issue as Reggit where the pipe didn't want to release from the EGR, but I didn't have the luxury of being able to cut it. I managed to get some silicone spray into the fitting and after about half an hour of wiggling it popped out.
In my case the seal on the original thermostat had perished so severely that it had detached and become wrapped up in the spring. The thermostat was completely bypassed as a result. I had briefly considered leaving it alone and fitting a secondary thermostat (a Rover PEM101020 looked like a good candidate) but with hindsight I'm pleased that I've removed the dodgy seal from the system. I'm not sure it'd do my water pump much good if it broke loose.
My car was running so cold on the motorway that it had started throwing P0116 codes. I decided to also swap the coolant temp sensor as access was easy whilst everything was apart and drained. I'm 99% sure that the code that it threw was because of the car over-cooling rather than a sensor issue, but in the unlikely event that there were multiple issues it seemed prudent for the £15 that a Febi sensor cost.
Reassembly was relatively straightforward, although getting the EGR back into position took some time. It took a few journeys for the coolant level to remain on maximum, but it did eventually and hasn't moved in the 350 miles since then. Coolant temps now sit at a constant 94-95c when monitored over OBD. I hope that's useful to anyone else foolish enough to attempt this.