Checked the water today and it was a little low again, it was last topped up at Christmas.
Also checked the oil and the dipstick was covered in brown sludge.
I presume the Head Gasket is on its way out, though so far there has been no noticable overheating and the sludge was not present after I wiped it off and went for a short journey.
If it is not the Head Gasket and water getting in, any other suggestions as to the sludge ?