I was just in Home Bargains and they had one for £7. It looks similar to some of the Amazon ones, but not as nice as the one aardvark5 linked to. The material looked reasonably strong, although the finishing was cheap as you would expect.
Probably worth a try for £7, and can always get a nicer one if and when tailored ones for the S5 become available at a good price. It seems like they are more expensive when the car is new, like the mats are a good £10 more than very similar looking ones for the MG4.