I finally decided that it would be best to let a professional deal with this, so yesterday I paid a visit to Vantuner (Martyn) who removed the speed limiter and remapped the van to around 120PS at the same time. To say the van is better would be a gross understatement; it's totally transformed, smoother and a lot more powerful right the way through the rev range.
Martyn also gave me a lot of free advice, replaced the pressure relief valve, deleted the EGR, and reprogrammed the central locking so that the bloody thing no longer behaves like a Venus Fly Trap when you accidentally leave your keys inside
He's also given me a list of other things I need to look at - a sloppy engine mounting somewhere, a top end oil leak (injector seals), and a bit of chafing on the heater hoses.
I can't recommend Martyn's services highly enough, he's a nice chap and really knows his stuff.