Finally flushed the heater matrix as I was fed up with the interior misting.
I've been meaning to do this for a while, and had already prepared a couple of adapters to plug the garden hose directly into the heater hoses. These were to avoid having to fiddle about with the bulkhead connectors.
I turned down a couple of lengths of aluminium bar to 21mm(to fit inside the hose), screwcut a male 1/2"BSP thread on the other end, and drilled a 12mm hole through the middle. These meant I could use standard Hozelock connectors to attach the garden-hose supply and a drain hose. A bung in the open end of the lower hose completed the set.
A few minutes of swapping the supply and drain hose soon flushed the matrix so that the water was running clear. I then added a bottle of dishwasher cleaner and left it to soak for about 15minutes. Flushed it through again, and reassembled everything.
Now the heater works!