See the thread 'Wich form of water and space heating' further down in this Topic. A few comments in answer to my original question.
I've also downloaded the manuals from the Alde website. From these it seems to me that the Alde system is more complicated to maintain (change of liquid every 2 years) and to set up - read the instructions for getting the air out of the system.
I'm a bit reluctant to have systems I cannot fix myself. The Alde also suffers from the problem that one failure will leave you without both heat and hot water, and is reported to be much slower to warm up the van. This matters to me as we travel down across Europe each New Year and need a quick warm up for the overnight stops usually on sites with low EHU power available.