Merchant Venturer
Please accept my apologies. I didn't mean my reply to seem severe. In answer to your query concerning 'Airfield Elevation vs. Aircraft Performance', it is generally when Airfield Elev. is in the 1000's of feet above mean sea level that Aircraft Performance starts to be seriously degraded(if I remember correctly Bristol is in the mid 100's). When it is coupled with high temperatures then it really starts to bite.
Hope this helps. Regards
Pontious.