Thanks for the replies, folks. A google search I did brought up some old threads here (2011) but not the more recent ones so I didn't bother searching the forum for check strap. Clearly now, I see it's a problem many owners have.
I realise that my wife's car is a phase 1, not 2, as it has the roll pin rather than the fixed clevis pin. From what I understand, the phase 2 appeared in 1998, so this is one of the last phase 1 cars?
I'm assuming I can get the Fiat Doblo unit as described in the other threads, dismantle it, change the strap to get it the right length, and refit. I'm hoping I can attempt to extract the old unit without taking the door card off but if I must, I'll have to tackle that seized screw first. I can certainly reach it with my hand and feel the broken flanges and the ends of the two screws inside the door...
I've since examined the nearside door mirror. Are the phase 1/2/3 mirrors all the same? This one does not have a painted shell, it's black plastic and so fits any colour car. So, any nearside mirrors from a breaker's will fit? Or are there variants?