They will be GU5.3 MR16s. They are low voltage Halogens, whereas GU10s are mains voltage halogens.
It will, unfortunately, make finding a good quality LED option a bit harder.
LedHut have five or six MR16 Led lamps, & offer a hassle free 30 days return, so you could try a couple of them & see what they are like.
Go through Quidco, you'll get a voucher & cash back.
	
 
 Just to clarify, the Philips LED's in Tesco are £5 each but it's currently 3 for 2. This offer has been on and off quite often over the last few weeks in my local store so it's worth keeping an eye out for the offer. 

