Post Code and HMRC

I will be posting off a 64-8 for a client this morning. Her daughter has power of attorney, and I have seen correspondence addressed to her daughter on behalf of the mother from HMRC, noting daughter is capacitor. I have therefore used daughter's address on 64-8 and noted she has power of attorney. I only have 2 months in which to file tax return (extension granted by HMRC).

I tried to register online as SA agent, but it fails. Apparently I must input mother's postcode - which is that of her nursing home. HMRC say I have the right address of this home, but the wrong post code, which of course they cannot tell me. Daughter has given me the postcode independently, and looking up the home on Google, I get that postcode. So probably HMRC entered the wrong postcode and that's that.

Have you had similar circumstances? Shall I just bang my head against a brick wall?