Well... I must say - credit where credit is due.. I phoned VM today to enquire about the delivery of my new 4g SIM card and was told it would be with me at 1200 hrs at the latest.. It arrived at 1153 hrs..
Texted blah blah blah etc. to 789678 and got confirmation that request for activation had been received..
Waited about a minute before switching off phone, removing old sim, inserting new sim..
Phone restarted with a lovely 4g+ icon in the top bar.