There is a file somewhere on the forum (posted by Linus, I think) that lets you download historical data from Betfair. I'll look it up if I get a chance.
However, from my research into the historical data I've downloaded, simply laying the field in-play of every market at a low price is a non-starter (even though there is loads of people apparently use this strategy).
Even laying at 1.01 no longer appears to be profitable.
Hope this helps.
Jeff