A while back I was getting monthly bills even though I'm quarterly (every three months) but thankfully it was sorted quickly and without bailiffs being mentioned! It was some moronic **bleep** at the Glasgow office that took it upon himself to change my tariff without as much as a by or leave! Anyway if this happens again contact Ofcom!. So don't sit crying about it! Take action and find out your consumer rights!
I can only suggest that you do the same as I did and move to another supplier as soon as possible.
Scottish Power's billing system is faulty and they won't fix it until its effect starts hitting their bottom line. The stress caused by their wholly unjustified threats to invoke debt collectors isn't worth the trivial savings on their tarrifs.
Take actual meter readings when you switch over to the new supplier as well.
@bailey From comments here SP seem to be sending bills at shorter intervals than quarterly in order to improve their cashflow. Also if a meter reading is submitted it will always raise a new bill regardless of a quarterly agreement.
@Paulshaw962 These estimates are often based on 'typical user' data and may not apply to your situation. The only way to compare is to look at the daily fixed charge and kWh rates for your old and new contracts. This information is always available on your bill statements. If you are not happy with your transfer you have 14 days cooling-off period to cancel it.