Hello, my name is Mark and I will try help you with this.
I would say you have been very poorly treated by this garage.
I would also like to give you some things to consider saying to the garage about your problem.
From the sounds of it, I would guess you haven't done a lot of miles since the initial repair was done.
This would mean after the engine rebuild you would expect the garage to have set the oil level correctly.
If they are now saying a lack of oil caused the new fault then either the rebuild was faulty or the engine has a severe oil leak.
Have you noticed any oil on your driveway or on the road where you park?
I would also suggest you involve your local trading standards office or send the garage a lawyers letter stating you want your money back or the engine repaired.