Update: Well there isn't one lol, yesterday I witnessed one or my Marans trying to get in the nest next to the one laying, she tried this a few times.
Then she went in the other nest, rearranged the straw to make the circle then came out, then back in, then tried to go next to the one laying again.
Then the grey one started to go in the nest, this is the one I thought laid the mystery egg, she too came out, after a few minutes of this they gave up and carried on scratching about in the garden, so I only had the one egg from the soft egg layer, which luckily I caught before she broke it getting out of the nest.
Moving on to this morning, the same thing all over again, it was like a replay of the day before, I again got the one egg, the others went back in the garden again, I'll keep having a look in case one of them decides to lay.
The one thing puzzling me is the Maran that was trying to get in the nest is the one that was laying regularly before she stopped about a month ago, so is it possible she would start again after the month long stop?