As promised, an update and some pictures - we've had nobody come forward, and have made the decision today they are almost certainly abandoned, as I haven't found any signs of anyone looking for them.
They were found in the road next to a big commercial dumpster/bin - someone dumped a mattress in it which was propping it open, and while we might be jumping to the wrong conclusion here, suspect they may have been dumped inside, and it was just luck that the mattress was put in as well.
Complete guess based on guides online, but we think they are around 10-12 weeks old, both boys. They have clearly been loved and cared for at some point, as they are extremely well behaved (as far as kittens can be!) and very sociable and affectionate - they love their cuddles. I can't fathom why anyone would dump such beautiful chaps as they have lovely little personalities already.
We are still trying to figure out what to do with them, the vet said letting the RSPCA take them is probably the sensible option, but honestly, I don’t know if I trust them, and regardless, it wouldn’t be fair on them being stuck in a shelter for months – I’m going to enquire about us fostering them until a home is found, but I think policy and bureaucracy is going to get in the way of that idea.
Still waiting for Cats Protection to get back to us, but again, not holding out much hope. We don't qualify for any of the financial help with cheap neutering, and they're likely to just refer us to the RSPCA as well, so getting them snipped & chipped is most likely going to come out of our pocket - not ideal, but still better than the alternative!
We'd love to keep them, but with a dog, 3 cats of our own (and 2 children), the house is already turning into a zoo, and I'm not sure we could handle 2 more long term "residents", plus our greyhound seems to be getting a bit stressed out; when they're chilling it's OK, but when they want to play, they nip at her paws and tail, and she's really patient with them, but has started just hiding in her cage looking depressed