Just an update - Zen support reset my profile and now I'm faster than ever before! Around 6.5mbps whereas previously before the issues I was getting 5.5mbps*
It appears to be stable, so hopefully it'll stay this way.
*yes 1mbps extra is a lot for us ADSL guys
Current on Zen, getting around 5mb down - .8mb up
Exchange is Fibre enabled, Cab not economically viable to upgrade - though 'Now Exploring Solutions aka we want someone else to pay for it.'