Warning: Dont let it fail to start too often!
Hi Nick, A couple of suggestions. Assuming there is plenty of fuel and it gives up trying to start after 30 seconds, then re-tries a couple of times and then gives up then I suspect the glow pin may have gone.
If you can do it easily measure the current whilst it is trying to start, it should be taking 7 to 10 amps (at 12 volts).
The real answer is to plug in the diagnostic unit and check the fault codes.
The diagnostic unit costs around £200 but it worth having.
Warning: if you try to start it too many times (ten?) without success it will lock-up (with error code 050) and then it can not be started without clearing the fault with the diagnostic unit.