Looks like lime mortar to me. Most likely on any pre1919 building. (1919 is when cavity walls came in) Where are you? Nationally you want NHTG for all things traditional building skills: http://www.the-nhtg.org.uk/
Just moved in to a house with a oil fired Boultler Boilers Comray Compact hot water and central heating boiler. It has a hot water storage tank with a header tank. Not sure if this indicates that is a sealed system or not.
The issue is the 'Limit' light keeps coming on and has to be reset...
