Looking once more!
For our trip later this year...
Ant's Ebay link above seems to have expired but, I found these 1000m units after a quick search:
http://www.ebay.co.uk/itm/2x-BT-Bluetooth-Intercom-Headset-1000m-Motorcycle-Motorbike-Helmet-Interphone-/351344651856?pt=LH_DefaultDomain_3&hash=item51cdc5 ee50 Recommendations still welcome.