NonprayingMantis: look at it this way.
you were getting a better service than expected before when there were fewer users than there was capacity. they were undersubscribed so everybody got more bandwidth.
now that they have taken on more people in the cabinet (and this will be a chorus thing, not telecom) the speeds have dropped back to the level that you 'should' be getting when the cabinet is fully subscribed.
they let motorways get congested at peak hours for years before they upgrade them. this is the same kind of thing.
sucks for you unfortunately, but that is the nature of things. sorry that isn't much help but there really isn;t anything that you, telecom or any other ISP can do about it short of you moving house.