I am back after an early morning start. Hard frost and -4 degrees, no wind. Set up first light. Magnet and some shells on field of OSR that has been hard hit for over a fortnight. However, Birds started to move onto another field in view. Cannot reason why that happened! But nothing much else moved for a while, until as you said, the sun shone and melted a bit of frost! But it was noon and I couldn't feel my toes.. so I packed up. At least I have learned a bit of field craft - winter osr and hard frost, have a lie in and wait for things to warm up!