If the stats are consistently good, then it doesn't seem likely it's a line problem that could be found/fixed by BT OpenReach as this would be reflected in stats from the router
If non-LLU then the kit that terminates your ADSL connection will be a BT wholesale managed DSLAM.
If both you & other customers are affected by slowdowns then it would suggest to me that the BT wholesale links from the exchange are overloaded - or that the DSLAM/line cards are faulty-- the ISP should be able to chase this.
Personally I'd not got for SFI. I don't have experience of the process, but it seems they would not find a fault and you could get charged.
I'd contact/push ISP, and in parallel consider writing direct to BT as someone else suggested, see if you can at least get some information out of BTw?
Edited by planetf1 (Wed 09-Jun-10 14:07:16)