Well thanks everyone. At least I know I'm not alone!
Don't think it's the weather. I'm in the far north of Scotland and whilst it's been pretty fair, 30C is but a dream!
Feed-wise, they get layers pellets (about 100g each) in the morning and a small handful of mixed corn in the afternoon. Out on grass all day, most days and the odd bolted greenery from the veg garden. So pretty much the same as you Waterbuffalofarmer. Interested in the seaweed idea. I have given them 'poultry spice' and the occasionally have apple cider vinegar in the water.
No squirrels or magpies and no evidence of rats .... although I'm sure there will be some about. There's normally someone here during the day, so nest boxes are checked regularly and we would hear any alarm caused by predators.
So, time to worm again - assuming I can find Flubenvet, which seems increasingly hard to get hold of. Anyone swear by Vermex? Never tried it myself.
Thanks for your suggestions ....