Thanks for the advice. He seems a confident hamster and he was fine with the full bedding change so I decided to move him to the Grosvenor today. I think he seems quite happy. He explored the cage fully and then went back to sleep.
The platform is the plastic one that came with the cage and it's very shiny/slippery so I'm going to get either a hemp mat or a sheet of cork to line it with. I also want to get a cork log or something to go on the floor.
At the moment I've put his blue plastic hut just by the entrance of the new house so hopefully he'll move into it.
The cage door locks didn't come with the cage. The door is very loose, so that any hamster could easily push it open (potentially even accidentally while bar climbing). I bought the door locks on Ebay (search for hamster cage locks and they'll be one of the first results).