you could pick up a 2nd hand radiator for about ?20-30, f**k paying for a new one! and if your really handy you could get one that has a working fan also already attached - 2 birds killed with one stone.
then its a few bolts and 2 water pipes to refit
id be very surprised if the head gasket has gone.have you checked to see if water is mixing with oil? just have a look at the oild cap. if there is no white mayo gunk under it. it'll also be running like a bag of sh!te