Thanks for the advice. To be honest my sister's been looking to get a MiTo for a while and this has forced her hand. Whilst I agree with you as regards the cost of repair versus the depreciation, it's her money and ultimately she'd fallen in love with the MiTo and nothing's going to change her mind.
I'll put it on ebay on her behalf and see what we get. She's got a pretty good deal on the MiTo - grey, red leather, fully specced up Veloce with 18" wheels, 2k miles, £15,000 exactly (was on for £15,995 and she haggled).
We'll be sad to see the 156 go, but for someone with the mechanical knowledge to fix it themselves they'll get a bargain, and good luck to them.