B,
I refer you to my earlier post. If you go to the HMRC website
http://www.hmrc.gov.uk/
and search under "Freedom of Information" or "Data Protection" then you will soon be able to determine for yourself the extent of the statutory protections afforded HM Revenue & Customs, in respect of requests for information from the general public or even taxpayers about their own affairs.
With regard to your mother's tax affairs, you are correct that HMRC can obtain a wealth of information, should it so desire, from other sources. However, your average Inspector finds it easier simply to ask the taxpayer.
I should recommend that you ask your mother to sign a form 64-8, authorising HMRC to liaise directly with you. Although your mother will still receive all relevant correspondence, you will receive a copy as 'agent' and they will be able to discuss matters more freely, with you.
http://www.hmrc.gov.uk/forms/64-8.pdf
You may also want to consider speaking to a professional tax adviser.
Regards,
Lambs