Is a lack of gas supply a common reason for the Potterton E133 error, and what should I check?

Yes, an insufficient or interrupted gas supply is a very common cause for the E133 error. You should check if other gas appliances in your home (like a gas hob) are working. Ensure your gas meter's emergency control valve is fully open and that you haven't run out of credit on a pre-payment meter. If other gas appliances also aren't working, contact your gas supplier.