Excel 2007 : Autofilter Colour Heading Cell with filter

I have code taken from another source (not sure if Im allowed to say what website I got it from - but incase not - I'd like to thank the author)

Basically it highlights the top cell in a filtered range, if there is a filter in place on that column. So far so good. The problem I have is that If I then open another workbook, its applying the same rule. I want the code only to apply to the original workbook. I believe the problem is that the code references activesheet and therefore is triggered by any other open workbook. How can I change the code so that it only runs on the workbook for which it was intended. The code is as follows:

In Thisworkbook

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and then in a separate module

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I've tried changing activesheet to thisworkbook but that returns an error?

Any help would be greatly appreciated.

Thank you in advance.