Thanks guys, sounds like I do indeed need to get this investigated further. I've been trying (unsucessfully) to get this resolved by Plusnet, my current ISP, but despite 5 months of fault tickets, repeated engineer visits etc it's just as bad now as when I moved in. Plusnet's customer service is appalling, can't believe they advertise this as one of their 'good' points!!!
Anyway it's only a few days til I switch the broadband to A&A, I'm hoping they'll live up to their reputation and get this sorted for me