I have used both freezemark ltd and farmkey, I found them both proffesional and pretty quick to book with. If I remember rightly freezemark ltd were a few pounds cheaper, so if you have several do be done you may save quite a bit. Sorry cannot remember the exact amount as each time I have used them I have had other people horses done the same day, both companies do a cheaper rate the more horses they do on the day.
farmkey - i think it was £55 earlier this year but goes down the more you have done! One was done in about 5 mins inc clipping the area. The technician was excellent with my youngster.
I used Freezemark and they did give a discount depending on the amount of animals that were done, some times they advertise and you can get a discount voucher as well , in some of the horse mags.