Check out the Cobra MT800. Available in multi packs on ebay. http://www.ebay.co.uk/itm/COBRA-MT800-PMR-TWO-WAY-RADIO-WALKIE-TALKIE-QUAD-PACK-/200818257225?pt=UK_ConsumerElectronics_SpecialistRadioEquipment_SM&hash=item2ec1b36d49
One advantage of these radios is that you can use standard AAA batteries (four per unit) so the batteries are widely available. Again large amounts can be ordered via ebay.
http://www.ebay.co.uk/itm/24-X-AAA-Duracell-Alkaline-Batteries-MN2400-Battery-NEW-Duracel-/150729194134?pt=UK_ConsumerElectronics_Batteries_SM&hash=item231828fa96
Ignore what they say about range. But if you are reasonably close together then you should be okay. Having the same model means no complications with different instructions.
The best we have ever achieved is 8km (line of sight) using Cobra radios. We quite commonly get 4 to 5 km in the Lake District when out walking. (None to very poor mobile coverage).
One tip - Aerials like to be vertical so hanging from the rear view mirror works better than radio being sat on the seat or on the dash-top.
Cheers,