Firstly, I don't think a 50Ah battery is large enough for the TF. Rover used to fit a 40Ah to versions of the MGF without electric power steering, but I am pretty sure the standard fit battery for the TF was a 63Ah.
Even so, a new battery shouldn't drain in 3 weeks. If you are confident there is no parasitic drain (0.04 amps is perhaps fractionally higher than normal, but not significantly so - should usually get 0.02 to 0.03 amps), either the battery wasn't properly/fully charged when it was fitted, or the battery is a duff one. Your reading of only 12.2 volts seems a bit low - a fully charged strong battery should be around 12.6 volts (after being left to settle for a few hours after charging). Immediately after charging, it should read something around 13.2 volts.