Beware, apparently Milton can damage your boiler, better to use a product specially for the purpose. I quote from elsewhere:
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Hi We have been advised by Truma that you should NEVER use Milton, or similar products, in your fresh water tank as if any gets into your boiler it will corrode the Stainless steel in the boiler and cause it to leak and fail!!
There are other products on the market, such as Aqua sol which will sterilise your tank without damaging the boiler. They are a little more expensive, but replacing a boiler is very expensive!
Aqua Sol - is a complete water treatment that treats 625 litres of water. It is tasteless and odourless and easy to use. Simply add 1 capful (10ml) to every 25 litres of water. Aqua Sol kills bacteria and viruses, removes tastes and odours caused by plastics, chlorine and metals and helps keep water fresh.
PuriClean 400g - Cleans and purifies all stored water systems. All water tanks, pipes and taps become contaminated with dirt, stains, bacteria and algae. PuriClean provides a simple, effective method of cleaning and purifying the complete water system. PuriClean is non-toxic and non-caustic. It does not taste, smell, foam or harm any components in the water system.
We have always used Milton, hence the need for a completely reconditioned boiler!!"