The DPA is the bane of my life! I sympathise with your predicament, as has been said this doesnt seem like an instance where the records shouldnt be released to you.
Have just tweaked your letter slightly:
Thank you for your telephone message left earlier today stating that you are unable to provide the requested letter as it came from another party, who does not agree that I should have sight of the same. You advised that in view of their stance, and as you do not consider this document to be 'your' data, you feel unable to release the document to me.
I am dealing with the other party's comment as a separate issue as I believe this contravenes the terms of the Access to Health Records Act 1990 and also the 1998 Data Protection Act.
However, in refusing access to the records you hold I believe you are also in breach of the Act. The file you hold forms XXXX?s Health Records with yourselves and I am unaware of any valid reason under the provisions of either the Access to Health Records Act or the Data Protection Act (under which you are deemed to be the managers of the data) to allow you to withhold the requested letter from me.
I therefore repeat my request for sight of the aforementioned letter in accordance with the provisions of the Acts detailed above specifically Section 7(1) of the Data Protection Act.
I consider that you are obliged by law to provide disclosure upon my request, and trust that your position can now be fully reviewed, and the requested information provided without undue delay.