Constantly delaying work.

I've had an tingling issue with Virgin which started on March 1st. 

Two engineers have now called and it appears my Virgin connection has been 'temporary' for your some time. This results in poor broadband speeds and an intermittent service. I have a promise of an engineer's visit every five days. It's now getting laughable because on the day they are due to complete the work - along comes another text with a promise to do the work in five days time. This is driving me mad. I've now waited five months for the problem to be sorted and I'm paying full price for half a service. 

Any ideas what I can do? Can I claim compensation for the poor broadband speeds?