You have forgotten that the OP said: "I was also looking at putting a room in the loft so another rad would be added (so that gets rid of the tank in the loft and the hot water cyl).An unvented cylinder powered by an an Advanta 18s will give you a much better system, and cost considerably less to run than a 40Kw monster.
Exactly the point I was making; and I would not call the 937 amorrismini said:A 937 with such a small water store will not give 20 litres a minute for very long at all. I think it drops to around 14 litre per minute when the store is cold.
Agile said:"medium sized combi"
The 937 is another example of the "here's the solution, what's the problem" syndrome.