I've been watching with interest today and there are definitely two birds visiting the nest. In fact both birds were on the fence together this morning but of course my camera was in the house. They both look the same though. I would expect the male to be bigger or fancier. I'm amazed at how much stuff they are finding in my garden to catch. I'll have nothing left soon.
I have spent a while looking at both the pictures I took yesterday and the top of the left leg is a bit less feathery on one of the birds. Could it be that I have taken a photo of both birds or am I kidding myself?
I've had a Google and young Black Redstarts look similar and the males look like the females in winter apparently. Is it winter still? 
I've told my neighbour he has them nesting in his barn and he's not to disturb them and in no way eat them. You know the french - they'll eat anything which flies, swims or crawls. He thinks I'm a crazy english lady. No change there then....