Bit out of my field but you could start with the auction houses here as they seem to have offered works by her previously. If they’ve done their homework, and yours is by the same Mary, they should have some info on her. Send them a pic (with a close-up of the signature) size and general condition and ask them if they think it’s by the same C Mary Hibbs that they’ve auctioned in the past, what they know about her, and what yours might sell for in auction - they’ll need that latter sniff of carrot to get their capitalist butts into gear ;-)

Nice pic by the way, had to scroll back one to find it.

Edit: Ach, can't get that link to work properly - email me if you want any more info.