David, thank you for putting my message on Twitter. I am pleased to say we have been reunited with our keys.
I put some flyers along the route and I have had a couple of calls, one lady called to say she found a key on Swanland Road on Sunday morning. I suggested that she takes it to Hatfield police station. If you think you lost a car key down there it might be worth checking with the police at Hatfield.
I also had a call from a man who lost a bunch of keys in Welham Green last Thursday. He thinks he dropped them by the shops, possibly outside the chemist.
If you found them please put it on the forum. Thanks