The official Renault guidelines is an hour, we are only booking them on hour waiting appointments although alot of customers are choosing to leave the car with us while they are at work and pick it up in the evening.
I wouldn't think you would have much luck just turning up, you would more than likely get turned away, they have recalled all the Clio 2's and alot of customers have waited a few weeks for their waiting slot, they may be able to squeeze you in but I wouldn't get shitty as you've effectively jumped the que. There is also currently a recall on the early megane scenics on the door mirror control unit, this is again taking up alot of waiting slots.
Most dealerships will be carrying the catches, but it is up to them if they want to replace it, there is no guarantee you will get a new catch if there is signs of corrosion then they may well replace it, if its in good condition they won't simple as that. The responsibility lies with the dealer so if they have any doubt they will replace it.
In terms of throwing the letter in the bin, I would imagine your insurance company will be expecting you to have the recall done.
Hope this helps.