I did something like this once when doing a self build, it was a wood burning back boiler, in a site hut serving a couple of radiators in a portacabin
Expansion tank was in the roof space of the site hut, pump was on the return to boiler, it worked quite well for a "lash up" for over winter use
My self if I was going to do a static caravan for permanent use, I would be inclined to go the continental way with a sealed system, for the heating side (does away with the expansion tank and pipework) BUT fit a safety valve like this [DLMURL="http://www.thermador.fr/www/product.asp?objectid=26704&objectkind=5"]produit[/DLMURL] so in the event of over heat the boiler will dump hot/boiling water to waste and cold water will fill the boiler water jacket, so avoiding a dangerous situation, I would fit a circulator on the flow with a pipe thermostat to control the circulator, which would be used for both the indirect cylinder coil, and heating
If you went the traditional UK way with expansion tank etc, I would fit a safety valve like this [DLMURL="http://www.thermador.fr/www/product.asp?objectid=26748&objectkind=5"]produit[/DLMURL]
My reasons for fitting a capillary type safety valve instead of a spring loaded safety valve with a rubber washer, is that I don't trust them to lift before dangerous pressures can build up in the boiler, and I have heard that the rubber washer can get stuck to the seating of the valve, and not lift when needed
