Hi SurferTim, I'm really grateful for your response. Your code has been running all afternoon with only one failure and I'm putting that one down to the power cut! (New Zealand is a 3rd world country by utility standards.) This is the longest I think I've seen any Google Docs code run and I'm super-impressed with its functionality. If it stays running for the next few days I totally owe you 50 beers!
Cheers,
Le Scare