Thanks to the effort of a number of T2W and TT members we are now 100% that this is an ISP fault. When you can't connect you never get close to our server (all logs support this) and the server and it's internet connection have been given a thorough check.
The usual error is 10060 connection refused error, or a timeout error (neither of which are being generated by the server).
Usually connecting via a public HTTP proxy (e.g. 203.160.1.67) does allow a connection.
There is nothing we can do. If you feel motivated have a go at your ISP until they do something, or they get BT to do something to fix it
edit: one thing that did come up was to try lowering your MTU value (i think BT are recommening 1430) but if that means nothing to you it's unlikely to help
edit2: and you should make sure your firewall is not blocking all ICMP traffic, cos this is a bad idea...ICMP type 3 (unreachable) packets should definitiely be allowed in
I changed my MTU to 1430 but that didn't work. I used that proxy in Opera but couldn't connect. I found a list of other ones here http://www.proxy4free.com/page1.html
One of those works. Now I have forgotten what I wanted to post about
Just to let everyone know, that as if by magic this is now working again for me (I use PlusNet). Nothing changed at either my end or the Tactical Trader end so it's a bit of a mystery what went on. Anyway worth having another try if you were having these problems.