ive just installed some underfloor heating cable in my kitchen but when i test the resistance i only get a reading of 13 ohms when it should be 45 any idea what could be causing the problem

the meter is  fine i got a 45 reading before the floor was tiled do you think the wire could be damaged and if so how do i trace the damage

If the cable was damaged you would expect the reading to go up, or go up to open circuit (greater than your meter would measure).

To go lower is odd, you would need atleast two points of damage and very wet adhesive to get a reading like that IMO.

Are you measuring end to end?  What reading do you get from both ends to earth?  What reading do you get when you short your meter probes together?