Vauxhall Envoy vauxhall astra van envoy ctdi cooling system mystery

Hello folks, Irecently bought a Vauxhall Astra Van Envoy CDTI 2006 from a ued van dealer and have in the past 2 weeks have topped up the cooling system resovoir with 4 points of water, there r no obvious system leaks externally, no breaches in the rad or hoses and the oil has not emulsified, the engine hasnt over heated and stays at a regular 85 to 87 degrees and only touches 90 when stopped in traffic, the engine runs fine and isnt running lumpy as in over heating, this is the first diesal engine ive owned, am i missing something, all appears well but why am i having to top up with so much water?, any ideas?. Cheers. Chris.