why could my Swift Flow 75 boiler not work after changing my stop cock?

Swift Flow 75 - 80 Combi Boiler

I changed my Stop Cock and now my Swift Flow 75 - 80 Boiler will not work.

My boiler piolt light works, the fan kicks in when a tap is turned on or the heating system is turned on. all radiators are bled and all taps work, just no heat.

The Burner will not ignight, I've made no changes to the boiler only to to the house stop cock (cold supply)