I agree with Bendan that if it works, a standard DECT phone is the best way to go. I have extensively tested mine (branded BT 1500) and achieve almost exactly 100m in clear-sight conditions, so you are right on the limits.

Have they tried other mobile networks..... in West Country (and Scotland) the low-band networks (O2, Vodaphone) often work better than high-band (Orange, T-Mobile).

Personally, I know what I'd do.......run a length of cheap 2-core bell wire to the chalet (at night if necessary and bury it just into the ground). After all, these days you can do what you like after the master socket.

Vic