Skywriter, thank you, I was thinking about using an array, as this is how I would have done something similar in Matlab or Python, but the whole spreadsheet thing adds a different bent on the whole affair.

Thanks very much for all your input, Ed, I was going to use your version, as that is spot on with what I was after, but i am going to try the autofilter version of Skywriter's, as this makes good use of the code excel is written in, which I am assuming will always be somewhat quicker than a scripting language.

Tell you what I will try both and give feedback as to which one was the most efficient. Watch this space...