Blimey, those are expensive! I just used this -
http://www.ebay.co.uk/itm/12V-30AMP-SPLIT-CHARGE-RELAY-KIT-SELF-SWITCHING-/200767111515?pt=UK_Campervan_Caravan_Accessories&hash=item2ebea7015b" onclick="window.open(this.href);return false;
£18, took 5 minutes to install and has worked a treat...