Hello and welcome to Just answer.com, Super mechanic here. Please DON’T hit accept until you are completely satisfied with the answer. If you need any more on it just let me know. About your vehicle.
So sorry to hear all that. I agree with you though, they should make sure that it's fixed before giving it back to you. It sounds like none of them are sure exactly where the leak is. One thing they should have done, (maybe still do) is to put some dye in the oil, (or trans) to see where it is coming out from.
Aside from that, the engine may not be breathing as it should and that will allow crankcase pressures to get too high when driving. In fact there was a campaign on that for some of them.
I don't see how shops can charge for work that doesn't fix the problem, (it seems to happen though) and a good reputable shop should fix it at no cost at this point. (not that they will) but you should see what they are willing to do.
V8 XJ Series/XK
If yours does fall in the VIN's it would normally be covered by the dealer. In anycase, they will want to be sure that the engine is breathing correctly and find out EXACTLY where the leak is coming from.
Thanks and again, sorry to hear about your dilema. SM