There seems to be a bit of confusion in this thread as to what needs to be or must not be CE marked.
As I understand it no boiler under 2 litres of any rated pressure is covered by Directive 97/23/CE and therefore must not be CE marked.
Boilers over 2 litres and up to 50 bar.litres and intended to be sold must be CE marked but can be controlled, tested, and certified by the constructor who must supply a declaration of conformity, test report, etc.
Only boilers over 50 bar.litres must be tested by an independent notified body.
This is, of course, all based on EU regulations. What will happen after Brexit is anybody's guess.
Good luck with your venture Chris.
Russell.