Have you checked you don't have an air lock somewhere? Your symptons sounded v. similar to ours (hot water but no cold - v. mysterious since the feed is off a gravity tank). A call to Aqualisa suggested that we try testing for that first - and a few gallons of running water later it seems fine. Following their instructions we took the fixed shower head off, connected a hose to that pipe, with the other end low down (draining into the nearby WC), turned the water on, and after some bubbling and spluttering it worked. We now have both hot and cold water! (For a while the hot vanished and we just had cold!). You never know ...
G Wales, September 2010