I wonder why it doesn't work though Paul, as the speed test part no longer uses the phone number. AIUI it even works on non-BT Wholesale lines.
Only the Further Diagnostics require the phone number, but the postcode + hose number or the Circuit Service ID ought to work.
I like this further demonstration of BT Wholesale's lack of understanding of English

:-
"For broadband circuits delivered into your premises using fibre wires ....".
At least they don't say "premise", which is being used far too frequently on these forums, on the Main Site, and elsewhere on other sites about UK broadband.
My broadband basic info/help site -
www.robertos.me.uk. Domains, site and mail hosting -
Tsohost.
Connection - AAISP Home::1 80/20. 200GB. Sync 72313/12530Kbps @ 600m. BQMs -
IPv4 &
IPv6
Edited by RobertoS (Mon 09-Oct-17 14:04:43)