slime gathering in my galaxy expansion tank?

I have a ford galaxy 2.8l petrol on an 's' plate & it is boiling over when expansion cap is removed there is grey slime in the tank I have had it in ford's but all they say is it needs the engine replaced at a price of about £4000
has any body come across this problem and how did you mend it.
every thing else is great so a shame to scrap it...help??