See the "walkie talkie experiences - anyone" thread. I've bought some binatone 550's, as has the author of that thread now, but have no idea how effective they will be. There is, however, info on that thread re. cheap or even free sim card offers available.
The 550's btw where quite a bargain, a twin set was 25 +vat but I got a quad pack for 30 + vat(on offer at Makro - don't know if you have access to one).
Just hope they work!!
Happy skiing/boarding