All the chooks have angry-red bums. Anyone got any idea what this is, and how to remedy it?

TIA.

I can think of a couple of possibilities if multiple hens are affected:

1)This could be scalding from "cloacitis' - infection of the cloaca.
Any of the chooks have poo stuck around there? Any dietary changes or antibiotics been used?
Cloacitis can be caused by birds that are too fat and get a skin fold that holds the poos back too much, or can be randomly caused by various bacteria sometimes contagious between birds. Yeast can also cause it, in which case it is given the name "vent gleet."
Treatment if suspected bacterial is clean up and antibiotics (topical iodine maybe, systemic if bad?).
If suspected as yeast related it is said (but I cant vouch for) that 1 teaspoon of vinegar per liter of drinking water can slightly acidify it enough to help get rid of yeast.

2)Alternatively you could have one mean hen biting everyone on the bum..

If you chase the parasite avenue, then you can buy "aviverm" wormer to do the chooks (over the counter at vet clinic, has dose on bottle, not particularly expensive) via their water or down the neck (much more accurate).

It is possible to use ivermectin type drenches in water but its off label - its very hard to get an accurate dose for a hen that way, ivermectin doesnt like to dissolve in water easily either, chooks vary how much they drink and so on.

Certain of the cat and dog flea treatments can be used off label as spot ons on hens too for round worms and mites (eg: revolution)  but it works out pretty expensive.

No problem, hope they come good for you.