Just thought I'd add my own experience to this, been with VM since 2011 in Kingston Upon Thames. Started out on their 50Mb package. Absolutely flawless, never had a bad word to say about it. In 2012 I moved to another area in Kingston, got myself a free upgrade to 100Mb. Since then the quality has been awful. During offpeak times I will regularly receive 100Mb speeds however as soon as it hits 6PM (the time everyone actually wants to use the internet) surprise surprise I get speeds dropping as low as 0.4Mb.
Have been on the phone to VM several times since July last year, have received 3 different dates for the fixing of the service in that time. November, January and the latest one is April. No BT Infinity in my area so I'm stuck with them. Have been getting my broadband for half price since about September time but whats the point? I can't even use the internet for any online games lol
/endrant
Ask for free, you are in my boat by the sounds of things. Although mines never as bad as yours tbh.