Zarjaz - I shall of course be contacting Sky initially, but they seem to think all is ok and aren't overly interested in line stats.
The point is, you cannot contact OpenReach directly. You
must speak to your service provider (Sky), and they in turn talk to OpenReach. You have no direct contract for service with OpenReach, and they won't talk to you.
If Sky won't do what you ask, then your only option is to change to another service provider who will. There are some niche service providers like Andrews and Arnold who push really, really hard to get issues resolved - but this is reflected in their service pricing. You have to decide whether the chance of a few extra Mbps is worth that much to you.
Changing service provider by itself won't make any difference to your sync speed of course, except for the fact that you may get a DLM reset when you switch over.
What does the BT wholesale checker say for your line - in particular what is the "handback threshold"? If you are above this, then you are probably not going to get much improvement anyway (and you may be charged if an engineer callout finds no fault).
Copper lines are funny things: there are many, many variables. For example, the actual speed reduces as the number of FTTC users on your cabinet goes up, due to crosstalk. Speed estimates are just that - estimates, from a model which only partly reflects reality. On my line, the upload speed estimate range from the checker is 5-7M. In reality I get 5-5.5M in winter and 4-4.5M in summer. Presumably the physics are affected by temperature. It just shows that the real world is a lot more complex than the model.