Heating not working - Potterton Profile 60el Boiler?

I've recently moved into this house so don't know much about the boiler or anything but today my central heating has suddenly stopped working (it was switched on and working but later I felt cold and when I checked it had stopped working and all radiators were cold)

The boiler "sounds" like it is firing up and working - there is a blue flame towards the back of the boiler but I'm sure there is normally a flame right at the front as well?

I have checked gas supply for other appliances, all ok and I appear to have hot water too

Any suggestions to what could be the problem and if I can fix it myself (complete novice) or if I should call out an engineer?

Thanks in advance for any help given