You could well be right toekneem, I hope so. I'd happily thank the BT team if they have managed to get it stopped but, unless they tell me so, I'll not know.
Well, I contacted customer support as suggested above and I was contacted by one of the mods on the board who suggested that I instead contact the abuse team directly at firstname.lastname@example.org which I did. I'm not sure there is anyone at that address as I've received no reply whatsoever to either of the two emails I sent them! Not even an acknowledgement!
So that seems to be it then, BT seem unwilling to help.
Thanks again to the previous posters who tried to help and to Patrick the mod who pointed me to the abuse team.
Sorry to hear about this distressing problem. The Abuse team may not actually give any feedback for fairly obvious reasons of security etc.
The abuse mailbox was "manned" a while back when I was sending them spam reports about compromised BT connections, however they started to use an online web form, which was frankly very unweildy and so I stopped sending them. The abuse team did used to reply saying that they would investigate.
If the abusive poster is actually using their own connection, rather than via a trojanned machine, then the IP address in use at that time will be associated with the "owner" of the connection. The various teams at the blog site will have logs of the IP address sending the posts and they may well inform BT of this.
As this is possibly also a police matter, the details of the user may be requested by the police from BT.
Of course, the "owner" of the IP address at the time of the postings may well be unaware that their connection is being used if they have an open wireless (or weak password encryption etc) - so clearly the correct miscreant may need extra finding.
It could be that the person is using a BTFon/Openzone access. This gives a unique IP address for that connection. However, there is a problem with this as well. A user who has had their username/password compromised could be inadvertantly be thought of as doing these awful messages.
It's not as clear cut as first may seem. It does depend on how the person doing this is hiding their tracks, if at all.
For peace of mind, i would mail the mods again, they may be able to get some closure on this.