i had the same problem on my saxo vtr, im sorry people i know its a disgraceful chavvy monster but i sold it

anyway... i bought some radiator cement for like £7, put it through the coolent system and let the car run for a good hour so the cement
gets hot, it solved the problem brilliantly, and this whole was like the size of a 50 pence piece so it wasnt a small whole really!
i would defo try that first, takes 2 mins to do and will save you alot of money if it works, you can get it from most car places