Hi Dave,
Would have thought that metal gaskets were once only use, as they are usually of the crush type and so form to the metal they are crushed between and form a seal.
For what they cost, it would make sense to change it if ones been loosened. But it sounds like this may not have been done.
Further tightening the bolts could cause damage to the cylinder head or stretch the bolts themselves.
I've just had to have the timing cover gasket on my Mandello replaced as it was leaking. I'd tried tightening the bolts around the leak area but to no avail.
The timing cover gasket is a thin metal type with a central ridge which forms a seal when it's compressed.
The problem was that at speed the oil was running along the length of the sump and ending up on the rear tyre which made for some interesting handling.