TalkTalk Business is completely separate from BT, I am on that for Fibre [resold] while my BTW line is dying a slow painful death at Zen.
I said this last year & this year, they have NOT allowed for enough bandwidth at the exchanges to cope with all the interconnected household appliances that are now in use today.
At home you have ipads,iphones,TV's, Sky HD, kindles.... hell even fridges connected to the tintetnet....
It's just going to get worse before it's gonna get better.
Some of the fibre cabinet are on a waiting list now locally for connections so people are having to wait for someone to give up their broadband before they can get fibre...eh ...DOH!!!!
Only bad thing for you is a contract is still liable for payment with Zen and I imagine they will have covered their bases for low throughput.
Fibre Line 1 BQM
ZeN Line 2 BQM
BT Backhaul sucks