This has been quite an enlightening process.
I have discovered the following:
1. You cannot submit a MAC code to the 'chat to us, y0' thing on the website due to... who knows
2. You cannot submit a MAC code via the telephone unless you are the 'account holder', regardless that your mobile number and email address may be present on the account. Plus.net take the data protection act very seriously although I'm at a complete loss as to how a third party benefits from providing a MAC code?
3. Apparently you *can* submit it via the Plus.net portal but I've not been able to get this to work:
All I have to do now, is get my deaf grandparents to call in and officially announce me as the account holder.
Job done!