Hey Mat
Wasn't actually suggesting chemicals but didn't know if you had any nitrogen source available, I guess William came up with a source everyone has available
We also have heavy clay soils here and I have had some great results mixing in aged horse manure.
As for the drainage holes I guess it's either that or the small stones. If you go with the small stones you could put some sort of drainage pipe in near the side thus enabling you to check the height of the water in the barrel by poking a stick into it, like an oil dipstick (hope that makes sense and doens't sound like an insult, my English isn't all that good)