You are paying for a service that you are not getting between 08.00 to 21.45hrs. Have you thought of contacting Trading standards or similar bodies as you don't wish to change ISP's?
It's not up to you to fix a product problem as you are the paying customer. Tell your ISP that you are going to report them for failing to supply you with the product you are paying for.
I'm with you there not wishing to move to another ISP so threaten them and carry out your threat and report them to trading standards. Also tell them about PCA forum where you've aired your views. Good luck.