Used to have an account on adslguide.org but everything has been going so swimmingly I've not visited for years

A lot of the time I get a Firefox popup warning (on iMac at the moment but happens with any PC) stating the following -
'SSL peer reports incorrect Message Authentication Code. (Error code: ssl_error_bad_mac_alert)'
From a bit of Google searching I have seen a handful of people on the official PlusNet forums complaining of the same thing over the last few days in Sheffield and most on the same exchange as me.
Anyone having the same problems or have any idea what to do next? I fear the standard support at BT will get me trying all sorts with my browser and computers when I'm rather confident they are not the problem. A few posts on the forum think it may be an exchange issue but who do I contact about that?
Browsing anything at the moment is proving to be quite a nightmare and speeds are down so badly as well.
Any help is appreciated!
Dave