Well, today was the big day - and no, I didn't check the date before leaving the house!
I spent quite a long time in my LBS, trying this, that and the other and eventually went for A530s for the Marin - I tried the M424s but there was too much pressing against my foot in normal shoes. And as it turned out, the A530s worked absolutely fine. I rode through town on the flats then flicked them over once I was past the last set of lights. They stayed put, no matter which side they were on, so there wasn't any silly flailing around trying to get the correct side. Nor was there - I was especially pleased to note - any flailing around due to one of "those" moments. Touch wood, the toe strap moments on the road bike have taught me my lesson. Having said that, I've already packed the first aid kit for tomorrow's ride.
I'm going to leave it a week or two before deciding which pedals to go for on the road bike, note quite got my head around that one yet.
Now I've just got to improve my confidence and then slowly start tightening the tension. Gulp!
Thanks, everyone, for your input and suggestions.