What type of foaling alarm are you looking for? The only one which is any good is the one stitched onto the vulva. It's called Foal Alert, can you get them in the UK? Doubt you could hire them though.
I have used the foaling alarm that uses dampness, 3 times now. It worked perfectly even though my mare doesn't sweat much. If I had just used a camera, I would probably have missed the actual foaling as she is very quiet. She would probably have foaled while I was making coffee. The most effective are the stitched ones, but I think you need a vet to put it in. I used www.foalingalarms.co.uk.
thank you for the info I will have a look at that. My plan was to use cameras and the alarm to keep an eye on her as she managed to foal twice before without us noticing! We are now in a much better situation, she is at home and I can see into her stable from my bedroom, but needless to say I don't want to miss this one, so was hoping to rig everything possible up to help ensure that we don't. I will also ask my vet about the ones that are stitched into the vulva, I'd not heard about them before.