Dear Andy

I am writing to thank you for all the efforts that you have made to trace me and the work you have done on my behalf..

I must also apologise for being a bit of a nuisance and writing emails to you all the time to hear about what progress was being made!
The amount involved was far more than I expected which was a huge surprise.

I have also written to Dr Nowak to thank him too.
Best wishes and kind regards