Hi all, just wanted to say "thank you" for all your replies and to update you.
The worst is definitely over, thankfully!
Two of the hens don't make any noise at all now when laying, and the other hen, Grizel, bu-bu-bu-BAWKs! for about 5 mins and then quietens down again. She's generally the noisy one anyway, likes to make it known that she's HUNGRY!!! But thankfully, nothing so bad that we have to consider re-homing them.
We fitted an automatic door, and blackout fabric (what you've got to do, eh?) to fool them into thinking it's still night, so they don't get out now until 8am, by which time most people near us are up for work. We also gave our next-door neighbours a key to the garden, and told them that if we were out for the day etc, and the hens were noisy, to just let themselves in and chuck in some mealworm/sultanas or something to keep the hens quiet. We've also been giving the neighbours some eggs, and they're now actually becoming quite fond of the hens, so fingers crossed, it's going okay and we can now relax and enjoy the hens - and their eggs