Whats wrong with my Vauxhall Astra 1.6 turbo diesel 1995?

At first the water system was losing water, then one hot day it overheated, i thought this was due to the lack of water so i refilled. Later that day it overheated again and again, so i called the recovery service out, they found a leak in a pipe and repaired it, but then it was noticed that preasure was building up in the water system and i also noticed oil in the water. What could be wrong?

Max
June 2007