The Second burn produced a bucket full of charcoal
The third burn produced a big bag full of charcoal
Well it took us four "burns" in total but we can now produce a decent amount of charcoal from a 45 gallon oil drum. The trick is to leave it longer than you think and when it is burning really hot put the lid on but wedged open a bit, leaving the air holes open for about 15 minutes till the smoke changes colour. Then seal it up and don't open it too soon (we opened ours the next morning and I realised that some was still on fire in the box).
We had a BBQ last night and it burnt really well (with no smoke) so I was quite pleased. Dave's on about selling it, but it is quite labour intensive to make, we'll sell some and see if we can make any money from it.