Looks like the Vodafone discount is going, going, gone.
Shame, I used to have two x Vodafone FTTC lines for exactly this reason, knowing both would sync well under the 25mbps I could chaim the additional 15% discount.
Wanted to provide an update. I just never had time to bring the connection down to try and force a resync. I switched to Vodafone just over a week ago and the sync has gone from 20mbps to 28.9mbps (and seems stable - it even went up from the initial 24mbps). Really happy as that's a near 50% jump in speeds which has made an enormous difference (It's also vaguely close to the up to 36mbps I'm always offered).
I definitely signed up while the 25mbps minimum applied so I better get it should the situation deteriorate in the future, however.
Glad I never took the second line though as work want me back in! Thanks for all the advice.