When you have the oil switched on JUST to heat the water, does this mean I'm using less oil? or does it use the same amout of oil as it would if I was heating the house.
Yes you are using less oil. I use about 5 times more oil in the depths of winter to heat the house and provide hot water as I am currently using to just heat the hot water, and my house is insulated to modern and current Building Regs, so a porrly insulated house might be expected to use even more as a ratio.