He has never done anything like this in the past, always coming into us if he wakes and I have no idea how we didn't hear him over the monitor. Needless to say, it gave us a massive shock to find him downstairs and I already know I'll be listening out for him all night tonight.
When he did wake up this morning and I asked him why he came downstairs he told me that he wanted to see his "favourite little pet doggie Coco" who he had indeed let out of the kitchen, where she normally sleeps, so she could join him in the lounge. He clearly had an eventful night last night.