Thank you Aquarelle.
From the symptoms it is almost certainly SDA virus. Directly it doesn't do much harm, but it makes the immune system very busy. All rats have a bug called myco that lives in their lungs - normally it doesn't both them, but when the immune system is down or they are stress, it can flare up. So, the SDA busies the immune system, the myco flares up, and suddenly any secondary infection can take advantage. Pneumonia is the most common outcome - I've just never seen pneumonia move so fast as it did in these two girls.
I know the likely source now - the lone girl I spotted in a pet shop, and impulse-bought. NO more pet shop rats for me, this is just another reason to add to the list. Apparently two other branches of the same (nationwide) store are associated with outbreaks of SDA right now
Fortunately, it has a cycle of 10-14 days, so if we make it through two weeks, then these five girlies will then be OK and immune (so moving them out, or new rats in, shouldn't be a problem). Most of the rat societies, etc, put a conservative 60-day 'quarantine' on it, so no cuddling rats other than my own for a couple of months - not too hard, as I don't know many 'real life' rat people
I may talk to the vet and see if they will give me antibiotics to cover the remaining 5 from secondary infections - they will probably want to see all five to do that, which is fine, though of course that then potentially makes the vets office a source of SDA for any other rats that go in. Mind you, whenever I phone up and say 'I'd like to book an appointment for one of my rats to come in', they then refer to me by name (as in 'Certainly, Mrs. Maizie, when would you like to come in?') so I don't think there are many rats seen at the surgery!