Probably the ignition block faulty or the button/ one of the buttons stuck down locking it on. That’s about all it can be really, I would suggest isolating it from the mains and giving all the buttons/knobs a good clean off and try it again.
If it persists it’s likely that the block is faulty that being the case you can look at a repair but it is gas so you’ll need a CORGI registered repairer.
😀 Alex was correct. I loosened the ignition button and gave it a blast from a hairdryer (not too hot as I didn’t want to fry it!) and IT WORKS – Thanx – saved me time and money.