So I have sent in my third letter. The first was ignored and the second only prompted the receipt of a three line response from them basically saying "we have nothing".
So, this one, while polite and formal, is strongly worded to highlight that they have an obligation to provide the data I have requested. If not, further action will be taken. ( i have quoted all relevant legal sections and documents that pertain to my right to this data)
I have sent it via registered mail so that they can not ignore it.
I will post more details and perhaps even copies of the letters soon.