Same thing has been happening to me. Right now I'm having 75meg download speed but last night it was down to 9.
It's been slowing down in the evening a lot recently. It's as though BT Infinity is oversubscribed. Either that or they are throttling my connection even though i'm supposed to be on a totally unlimited package.
Thats intresting, your on P Dock exchange, me being milford you would not think there to be a link, that is exsaclyh my point, i pay BT for a premium service, i can get the same product through Talk Talk for a lot less, they need to sort it or ill cancel my contract based on them not providing the service