Sudden speed drop

[OldJim61]

I have Sky Broadband running off the old phone line (Openreach box is at the end of the street). I usually get 50Mbps+ (and have done with each of my previous providers). In the last week, my broaband has dropped and is now 25Mbps consistently. It seems stable and Sky tell me that this is within their promised range, but I would like to get my old speed back. Is there any way of knowing whether the problems is with Sky or with Openreach? I don't want to switch providers and find that Openreach is the problem. Thanks

[OldJim61]

Every time I run the Sky diagnositics they report "there's a problem with your broadband" and offer to fix it. I follow their steps and then get the same result and the same useless message. The only "solution" Sky are offering is that I can switch providers!