Would a 20 litre hot water urn fill a bath?

If you like really hot baths up to your chin, 20 litres won't be enough. But you could get two urns. Remember that you won't be running them nonstop so it should actually cost less than running a regular boiler. You can use one in the kitchen when you don't need it in the bath and you can also use them for parties and to loan to (really good) friends. The three I have have worked well for decades and are useful for all sorts of things. I may never replace my boiler.