Oil Filter Mount
 

Mechanic said the "oil filter to block is leaking oil" and that the block would have to be replaced as the gasket is not replaceable - sound right to you?

That does not sound right at all

nope, there is a gasket on the oil filter, the only thing that needs to be replaced other than the oil filter is your mechanic

I think he was referring to a piece that was mounted on the engine block and the oil filter screwed into it, and then something about this piece was leaking and that it would have to be replaced as its gasket was non-repairable - is there such a piece?

I'm quite sure the only way anything like that can happen is from a cracked block

Get a new mech. - this one is trying to live off your wallet. Isn't this the same mechanic that tried selling you 3 transmissions?

Gasket and more importantly the O-ring under the mount can be replaced. The filter stands can leak at the pressed in plug (mini freeze plug). Get more info from the mechanic. He might be referring to the plug.

as everyone already mentioned, you do not need a new block, that is completely horrendous. This guy only wants your money.