I know, Routem (as a side note to this thread). They tell you that the fare 'should' be no more than £25 to Egham/Staines, in fact the price list is actually printed, but they have a get out clause, if you like, that the driver has the final say on the fare. I was categorically told by the thief that if I didn't want to pay that amount then I had to get out of his taxi! I'd just got in from an overnight flight from Singapore, via AMS, (long story). I was just too knackered to get back out of his lousy cab & search for a reasonably priced ride home & it was being put onto expenses.
Unfortunately it's just another example of the completely crap, rip-off, frustrating experience that is Britain's premier ariport.