I went for this onehttp://cgi.ebay.co.uk/Diesel-Tuning-...|39:1|240:1318

but there are plenty others doing the same sort of thing.
It took about 10 mins to fit, you need to take the engine cover off to gain access to the fuel bar sensor (sounds easy but the strut brace is in the way) and then find somewhere to tie wrap the little box to, I put it next to the washer bottle so I can still adjust the screw on it's side.

You will loose some warranty, I took mine in for service and they made it quite clear that they noted the "modification" on the cars history and should any problems arise this will be taken into account... best just remove it before you take it to the dealer. As the chip does not plug into the ECU in any way they can not trace it if it has been removed.
I did read somewhere that this type of upgrade was not recommended for cars under 40,000 miles as it wears out the injectors quickly due to the higher pressure, but have since failed to find any other shortcomings.