Thanks Milo, and others. It seems that BT will be happy to keep charging me £1.60 a month for the 'premium mail' service even without an associated telephone line.
I have received a phishing email about this upgrade, and to warn others, here it is :
Image too large - 1024 x 800 max
The link goes to
http://iohodhd.net (link deliberately broken) ne.net/klkjndslf89745/ which obviously has sod all to do with BT.
The headers show that it comes from
Sender41.138.188.41
unknown
Nigeria Lagos
6.45313.3958AS37127 VISAFONE
There's a surprise!