Whenever your insurance term ends, the insurer should send you the proof of however many years you were driving under their cover without having an accident.
You would then normally pass this to your new insurer, so if you left Chruchill with 2 years, passed this to Admiral, continued with them for 2 years, at the end of the second year with Admiral, they would give you four years.
Sorry if that sounds patronising, but I'm just ensuring we have the same idea here. You did pass the proof on to your new insurer each time?
You say you've swapped insurers three times in four years - did you swap part way through the cover, or did you swap once the year's cover was up?
The only real way of obtaining your proof of NCB would be to contact each individual insurer and obtain confirmation from each of them as to how much you earned with each of them.