When does it overheat? The fans will only work in stationary traffic or if the car is sitting still idling somewhere for a period of time, as otherwise there is enough airflow to negate the need for them (they'll possibly kick in on extended track use as well). If it's overheating on normal driving, then I'd suggest the problem may lie elsewhere.
As for £700 fans, since you've got no warranty I'd be finding out the specs of the stock items and shopping the internet for a similar item (as long as the size/air draw/current is the same, there's no reason you can't fit one of those). Where you'd find that spec is a different matter and one I don't know, sorry.
And welcome to the forums, just sorry your first post here isn't a happy one!