Thanks for all the input. The Fontan are indeed cottered cranks, and I'm optimistic about getting those loose and off; the replacements, coming from the Peugeot, are what I was concerned about. I will find the 23mm tool recommended by others, and give that a whirl. If not, then the compromise will be to leave the crank on, and just replace the chain ring (existing rings are badly bent, hence the project in the first place). The chainrings are three pin (?).

Cheers,

Nick