Simon - you are going to have to get that "stuck" lifter out somehow. Maybe if you pulled the cam it would come out by pushing it out downwards? ( I can't remember the exact layout of the iron around the base of the lifter bores).
Then you can inspect the lifter bores (which should clean up with a hone, but that is going to mean complete engine strip down and clean to get the abrasive particles out) and the lifter, which sounds as if it is mushroomed at the bottom end - probably didn't get changed out for this very reason when the engine was rebuilt???
I can't see how else you are going to fix it - if the valve spring pressure wont push the lifter back down the bore it has serious problems.
Boy - you are going to be good at Ford engine building by the time you are finished!!
Bad luck old chap (that bit was for the benefit of our American chums)
