if there is just mayo just in the headder tank its puorous head if its in the headder and on the oil cap it will be the headgasket, you can get the head fixed for just over £100 by V-tuning, thats who did mine
i've read the guide to identifyin a porous head- it's definitly not a brownish/ orange colour to the sludge. It is white/ grey coloured- u reackon it could be some form of condensation?
I only bought the car approx 1 month back, the coolant was clear blue back then.
I changed the oil about one week ago- used 15/40
I havn't changed the coolant as yet- that'll be my next job, i can then see if & when the sludge will re-appear