Well, first thing would be to get it done at the same place for both runs
+1 hence why I went back to the same place as before. Confirmed none of their equipment had changed and they loaded up my results from last year to compare temps, conditions etc etc and they were pretty much exactly the same so could confidently discount any major environmental factors effecting the results which is why I am confident I got a sound reading to compare to last years.
Good thread, Andy. These things can never be a precise science but it looks like you've done as much as you possibly could to give a relevant comparison.