Not sure if you can change your mind after the bailiffs have been threatened.
your not changing your mind you state you never refused to pay just couldn't pay it in one go but the installer/builder what ever wouldn't except the offer.
The courts will want to do a financial assessment income and outgoings have to be declared and then the courts will agree on what is a reasonable periodic payment. Only individuals on benefits usually get away with a £1 a week.
don't you believe it by the time you've spent out gone through all the agro that one failed non paying customer can put the average self employed builder etc into serious credit problems.
and the easiest way to get round it is don't have the work done in your name.