It was around £150, I think £150 something and a few pence.
Another update anyway. I decided to ask PayPal about it again, they say they have looked at the account and there is nothing suspicious about it and they have no reason to believe it's not a genuine mistake on the senders part. They asked me to send it back using personal transfer friends and family to avoid fees. Got to be honest I'm not comfortable doing that. I expressed that to them and they said they guarantee it is safe to do. If something were to happen though their word means nothing.
So hang on, Paypal are asking you to refund either in a way that costs you money, or in a way that offers no protection if this is one of the well known scams where you are left responsible for the charge back when the original deposit turns out to have been from a stolen account/card?
I'd just sit on it and let them sort it out at their end. Paypal want to be so much like a bank, they should act like it and sort out these kinds of mistakes behind the scenes without expecting innocent customers to do it for them and end up exposing themselves to well known frauds.