My other car is a MINI!!!!
hope this isn't a silly question, but did you check the oil level after refilling mate? was it up to full then?
Oil drain from the turbo is a very likely cause, especially as it wasn't gasket'd before.. Very unlikely for you to have lost all the oil through a sealant issue with the sump / block join, because all the oil sits down in the sump, so wouldn't leak out of that joint anyway expect a bit when running.
I doubt you'll have to take the sump off again, but if it's the oil drain, you'll have to drain the oil again to get it clean and dry to re-seal!