I'd lift the tank,& check that nothings holding the throttle body itself open !you never know,there could be a teenager on a scooter stuck up it,they're f**king everywhere round here !

:sing:

This happened to me about a year ago. It turned out to be the foam inside Air Filter (aftermartket - without an internal support frame) got sucked in and jammed the butterfly open. It would only happen after a blast with the throttle wide open and would un-stick by blipping the throttle a couple of time's to release. I however found out the hard and expensive way what the problem was - still have the scars !