That is telling you that it is likely that there is a fault on the phone line, a phone line fault would explain such a dramatic drop in speed.
WIth that, if you call 133933, speak to tech support, give them the reference number from that and then they will be able to see what you have already tried and the result of the line test and will be able to go from there on getting the line fault looked into and hopefully fixed.
WIth that, if you call 133933, speak to tech support, give them the reference number from that and then they will be able to see what you have already tried and the result of the line test and will be able to go from there on getting the line fault looked into and hopefully fixed.