Welcome to TM Richard. I also occasionally get the seatbelt and handbrake warning light on, seemingly for no apparent reason. It usually reverts back to normal after the ignition is cycled on and off a couple of times. The handbrake was definitely not on and the rear discs remained cool after driving.
I also recall a continuous warning noise if the indicator lever is left switched to either left or right when the ignition is switched off. I've also occasionally had a ticking click, like an indicator repeater, when the ignition is switched off. Couldn't find any reason for it and and nothing was flashing on the dashboard, it eventually stopped after about 30 secs with ignition off.
I also get a random orange engine check light on, this is linked to the petrol emission system and the sealing of the petrol filler cap, code P0442. I now carry a OBD reader to double check the code and delete the error as and when it occurs.
The electrical reset as described by Asbojohn above is a good option if cycling the ignition fails, but do remember to re-programme your electric windows after, to ensure they drop and close again on opening and closing the doors with the hood up. Lower each window and hold the switch down for 10-15 secs and repeat when raising the window. Again, I don't think the fuel will cause this issue, I pretty much only run mine on E10.
