Hi Big Trev,
What a pain in the arse it was to get the Pressure relief valve loose, From memory it was a 19mm spanner for the main body of the Valve but using the open end is no good as the spanner will start to spread open before the valve will loosen.
You need to get the end bolt out that the Fuel return line is attached to using a 14mm spanner, but again it's very tight.
I also had to cut the 14mm spanner shorter as the firewall and various lines are in the way, then you need to hit the spanner with a hammer to break the bolt grip, once you have finished swearing & cursing & the bolt is out you can then put the ring end of the 19mm spanner finally on the main valve body & again use a hammer to crack the valve away from the fuel rail.
This took me four seperate occassions to finally get the bloody thing off!!!
Also make sure the spacer you put inside the pressure relief valve is about 4mm long as this is the recommended size to use.