Thanks for all your replies, it's much appreciated!
Regarding the fuel cost difference, I get this with the R (and so probably out of the question for me). But the overall cost of running a GTI against a GTD (fuel and monthly payments) shouldn't be too different, although I haven't done any detailed arithmetic to prove this!
I also get a car allowance from work which will pay the monthly payments of the finance. With discount, the GTI comes in well cheaper than the GTD even before the loyalty bonus (I also get the impression the loyalty bonus of £1000, correct me if I'm wrong, only applies to the GTI and normal golfs and not the GTD, GTE or R). The APR is 4.9% on the GTI against 6.4% of the GTD and the GMFV is more on the GTI (essentially making the payments smaller than that of the GTD, although I will have more to pay at the end if I choose to buy the car). So the offset in price for the PCP will in theory to some extent, offset the extra cost of fuel for the GTI against the GTD.
My local garage as also told me that the GTI is potentially a target car to sell, so the loyalty bonus may be able to be bumped up to £1500. Including the current discount on Drive the deal, This makes the GTI 5dr PP DSG in White with Pan Roof and Nav, £25,007.79. This is a saving of £6,746.97 off list price.
Hope this makes sense?