I had both the G & T boxes on my TD5 110 Van exchanged by Ashcrofts about a month ago. Cost around £1600 fitted (this included the 1.2 gearing). The work was carried out by Autoland 4x4 near Milton Keynes - so it will worth asking Ashcrofts if they outsource work to anyone down your way.
My Van previously had the 1.4 gearing + a GKN overdrive. Giving the engine a very good thrashing, top end was around 60 to 65 mph without the GKN, and with the it on, around 80 to 85 mph. The best mph I achieved with the GKN & a light foot was around 38 mpg.
I'm very happy with the 1.2 conversion, and would recomend it to anyone with a 1.4 box. Pull away from traffic lights for example is much more relaxing and you aren't in 3rd by the time you've crossed the lights, nor do you have the hassle of the 30 mph limit of the GKN. Top end is slightly down on the GKN - at a push around 80 mph. As you're probably aware, the 1.2 does not affect low ratio, and on the TD5, the anti-stall still runs at 4 mph (which is fantastic for traffic jams!!). Not too sure what the mpg is, but I'm probably down a bit, as I've also fitted BFG's 265 75's vs my old Avon 7.5's.
I have also read that some of the TDi's came with a 1.9 ratio, so if you do 95% road and no towing, this might also be worth considering.
Cheers
Tom