been having problems with it starting   and when i do  get it running  keeps cutting out untill its warmed up 

yesterday {very cold} i couldn't get it started at all 

back fired twice   sounds like it wants to start but then nothing 

{engine is actually in a vw type 2 bay }

any help much appreciated 

Are you running the stock ECU?

If so you might be able to use the ECU self code check function. 

If not I'd start by cleaning the idle control valve/maf and clean n re gap the plugs .