Dinky - Cab 18 here. I know of customer of mine on Cab 27 and cab 7 with similar issues - not quite as grim as my upload but has dropped from 19 ish to under 5. One is a business customer and one residential. And again like yours everything was fine last week and somewhere over the weekend it all fell apart.
BT openreach told me it may be a throughput error at the provider end but have elevated for further testing. An openreach to premesis througput test was done by the engineer which achieved a solid 14mbps (would of been more but during investigations the ip profile was downed from 20 to 14 and 79.9 to 60). Like you my connection is rock solid normally.
The bt speedtester gets around 3 and any other real world test gets way under 1mbps. the quality is so poor on the upload not even a single line sip phone is unable to handle decent call quality. I was told I would get an update Monday AM so will post any info in the hope it will help someone.
It appears it could be a BTWholesale issue from what has been said here. No doubt they'll be working on it.
Dinky Doughnut - just checked. Mine's CAB 51. It's sited about 100 metres from my front door. In early February Openreach engineers were working on it intermittently as the extensive changes to the Petticoat Lane/Spread Eagle works were being completed.
My upload speeds remain dismally slow. Have lodged an official complaint with BT (though the complaint page on the BT website was mysteriously unresponsive and I managed to pick up a link via a Daily Telegraph article from last autumn which enabled me to get off the appropriate e-mail). BT immediately responded with an automatic email telling me that I would hear from their Complaints Team within 10 days. Still no word from BT on the original logged fault which the BT site itself identified as a broadband problem relating to my service. I have a fault number, but everytime I access the BT site to see how things are progressing it displays a flow-diagram showing the investigation has not yet left the exchange! Since last posting I have used an ethernet cable to access the broadband signal directly. But this had absolutely no effect whatsover on the painfully slow upload speed. This is all very strange: until last weekend my upload speeds were wonderful - and had been consistently so for years. Whereas before I could upload a 20MB image file somewhere between 25-30 seconds, it now takes at least 45 minutes.
If you think it is an Openreach problem then you can use THIS link to lodge a complaint.
You would only be charged for an engineer visit if the problem is inside your home after the master socket. You can rule this out by connecting to the test socket with a filter. Whilst connected this way check there is no dial tone on any extention.