no, i am worrying because i find the level much higher than i left it before my last tour .
And much higher than the maxi level indicated on the bottle .
i've found the thermostat housing empty when the bottle became full
I am afraid to say that you are right to be concerned Laurent.
Dolomites (and some other BL cars) were fitted with a bottle that is called an "expansion tank" but
isn't by definition such a thing (an expansion tank is where coolant will go when the pressure cap is activated).
This is very much the weakest link in the cooling system. The drawback is that, as you have discovered, coolant level in the tank will stay
whilst the coolant is leaking elsewhere.
This is further compounded by the sender's location permitting the sender to be cooled by airflow, thereby fooling it to maintain a
normal reading despite coolant loss. (BL issued a warning about this in the workshop manuals.)
The good news is that it is perfectly practical to update to a header tank system to avoid this problem
(as has been the case since the 70s). (Triumph did so on the TR7).
Obviously Laurent, you will have to repair/replace the water pump.