sadly, they’ve been alone in a house for over three weeks, with food pushed through the letterbox
There’s a story there. It’ll be rapidly apparent how important this is.
So getting the cats into a carrier is likely to be a challenge.
Correct, They may be very nervy and unaccustomed to human contact. Earning trust is important here.
Besides taking thick padded gloves, any tips for catching them, please?
Honestly? It’ll be a battle unless you know what you’re dealing with first. Go in quietly, give them a space to retreat to if they need it and take treats. If they let you feed them by hand and they’re affectionate then getting them into the carrier won’t be an issue. If they’re hostile or very shy then it’ll take a while for them to get used to you. Avoid eye contact, talk to them a lot, leave cat biscuits and let them explore on their own terms until they trust you enough to put them into a carrier.
Believe me, even the most well adjusted and socialized cat might go loopy in a cat carrier. We’ve one who’ll always empty his bowels after a hundred yards in a carrier when we take him for his annual jabs.