Can be difficult to persuade an ISP to escalate it as (A BT engineer told me)they have to pay BT for any fault finding.
The issue is not so much the fault finding itself, but the fact that the BT monkeys like to take the easy route and just log a no-fault after probing a connection in their right nostril with their index finger for a couple of seconds.
Once they log a no-fault, the BT Billing department automatically raises a charge on the ISPs account. It then takes some persistence to convince BT that (a) the fault really exists and they should look harder, and (b) they should issue a credit note against the incorrect charge. If your ISP isn't that good, then they'll just sit back and take BT's bull and charge you accordingly any costs they incur related to your connection.... a good ISP will have a good go at fighting the BT behemoth into action on your behalf.
LLU operators are in a better position than the pure BT resellers, because they can troubleshoot a lot more in their own control before having to escalate to some BT monkeys. Hence I always encourage people to sign up with a LLU operator and not someone who is just rebranding BT's poor service.