Cancelling a contract due to speed issues?

Back in August (I believe date is relavent due to offcom reg changes) I changed my TalkTalk internet to FTTC with 18 month contract.
The estimated speed was 7 to 11MB,
After a 'bright sparks' and two OR visits to try and get improvements, my speed has settled down to (averaged over last month) 3.82MB, this is roughly 3 times what I was getting on ADSL, but thats only part of the story, whilst that is the raw speed figure I often suffer from errors which seem to result in delays in pages loading, so whilst a file of several MB will download quicker, individual pages whilst browsing are hardly any quicker.
If that wasn't bad enough I have been charged for the engineer visits dispite the fact that no fault was found in my home, the 'brightspark' engineer had logged a line fault, the first OR engineer had somehow cut my internet and blamed the router, the second OR visit was two weeks later and managed to fix the fault the first OR had made, and I was promised compensation for loss of internet, this of cause never happened.

So I'm now left paying a fibre 'premium' and getting very little for it.
After an online 'chat' to TalkTalk tonight I have to phone customer service tomorrow to try and sort it out. My thoughts are that TalkTalk have not provided the service they promised and either I should push for a better discount on the contract(the present offers are better than I get) or be returned to ADSL.
Does my speed of 3.82MB compared to promise of 7 to 11MB give me any leverage? vis a vis Offcom regs?

When you say estimated speed was 7 to 11MB do you mean 54 Mega bits per second (Mbps) to 88 Mega bits per second, or more like 7 Mbps to 11 Mbps.

If the latter then at those speed ranges it suggests you are around 1.5 to 1.8 km or more from the cabinet and estimates are more inaccurate at those sorts of distances.

Dropping to ADSL may be cheaper but is a speed 1/3 of what you have now i.e. 1 Mbps rather than 3.82 Mbps a speed you can make use of?

It is down to negotiation and switching provider is unlikely to get you any better connection speeds, if that is the root cause of the slow speeds, i.e. first diagnostic step is to log into the router and find out what its connection speed attenuation and noise margin figures are.

Andrew, I have been on 1.2Mb ADSL for years so know what it's like.
When FTTC was first available to me it was estimated as 4 to 5Mb, I guessed 4 would be probably max so would be paying at that time around four times the cost for three times the speed, I didn't like the figures. After 6 months the estimates changed to 7 to 11Mb, when I queried this I was told the revised figures came about due to experience on lines in the area, having used 7Mb ADSL connections I thought "that will do me".
Now I find the original 4Mb was closer to the mark, and the local OR engineer who knows these lines like the back of his hand confirms he would not expect anything faster.
As in my OP, big files defiantly download faster, BUT, for general web browsing I would say it's slower! This appears to be due to the amount of errors and establishing a link to any page (if thats the right term), I mostly use an old laptop and I was beginning to think it was on it's last legs as I would click on links and nothing would happen, or there would be a long delay, then gf started to complain as she was getting same problem on her much newer laptop, I now believe it is a symptom of being on the edge of usable VDSL.
This leaves me in the situation of 75% of my internet use being as slow or slower than ADSL at a greater cost. The only real 'hardship' will be once a week or fortnight when I watch a program on iPlayer, will have to download it to laptop and then connect to tv, instead of watching immediately on tv, well I can live with that.

So down to negotiating a way out of the contract then