All good info thanks.
It's just a crack that has been glued (not by me) before. I intend to take the gun apart and inspect it. To be honest, it hasn't been used more than once or twice since it was repaired and I wanted to see how it is and I'm anticipating what might need done, perhaps with another bit of glue on the inside. I can't see any evidence of a peg/dowel from the outside. It's a Sabatti O/U and compared with other guns has a pretty thin neck, which is where the crack is (was).
I might take the stock off and all is good. I suppose all depends how good the repair was done really. There aren't any replacement stocks for this gun available and to have one made is financially a non-starter, it's only worth a couple of hundred at best. So really I suppose it's make or, ahem break for this gun (sorry).
Thanks folks.