Baxi Solo 3 40PFL Problem?

Got a Baxi Solo 3 40PFL Boiler.
When turned on for Heating, it does so, and the rads get hot. But after say 20 minutes, the boiler turns off and will not come back on.
You have to switch everything off, and wait about 40 minutes for it to fire back up.
When calling for hot water though, it will run fine.