Hmmmm, you may have got me there FFF....Not sure about the drainage details, but agree that the field stays usable despite wet weather. Grass is good: if God had intended light aircraft to use hard runways, he would have given us better glue for the instructor's dentures.. .. .To draw attention away from my anorak malfunction on the drainage system, I'll venture to suggest that Waltham probably did have a FIDO system during the war. Being so near the meandering part of the Thames, there are occasions (eg most sunny mornings) when it could do with still having a FIDO. Have you guessed what it is yet?