You appear to be as baffled at my lack of success as I am, John.
Yesterday, second attempt, I followed Jason's path exactly and once logged on,arrived at my account details.
'edit account details' -> address duly edited ->save -> response = 'success, your account will be updated in a few minutes'
Also, under the 'RENEW' tab, 'change delivery address' changed/saved there also – same response.
Checked today and old address is still in place, it's only the first line of addr that needs changing.
Thanks for the help/suggestions, will send an e-mail to help@me.secureorder.co.uk after the weekend holiday
Cheers
DaveD