Your insurance co is quite right, a system in a van is a vehicle modification and you have to have it declared specifically on your motor insurance policy otherwise it is void, and you are driving without insurance.
Its also no good just telling them about it, as that doesnt count for anything. They have to actually ammend your policy so that the actual wording "VEHICLE MODIFIED TO INCLUDE 600 LITRE WATER TANK" or similar on your insurance certificate. If you dont have that then you're motor insurance in void.