Stick to using the BT wholesale checker with your phone number.

Thanks for offering to help Mick - I think I know what's happened.
BT have only enabled a limited number of ports (if that's the right word) on the cab I'm on and I've been told by a BT employee that the message I see will occur when there's no more availability on the cab. They've confirmed that my postcode is in a FTTC-enabled area but none of the individual addresses can now get FTTC.
If I can figure out how to send you a PM, I'll send you my phone number so that you can confirm this.

And an update - just been onto the BT Infinity site and my line will be enabled from 28th April. I'll be getting in my order with Zen sharpish.