I used Dartford Transmissions when I had mine done. It was at the peak of the fuel/oil prices at about £585 all in. They did a flush on it too so you end up throwing away about 3l oil once you've run it up and then dropped the pan to change the filter. That was a ZF6 and a couple of years ago. I need to get my ZF8 in to be done too as it's got 90k on the clock and never been done to my knowledge. However you look at it, it's not a cheap job but that's life eh?
At £450, I'd be bloody tempted to drive up to see Graculus for it though even if he doesn't want it driven in hot!