Toffee is much better and almost entirely back to his usual self. As it happened, the vet was coming out to the yard yesterday to see another horse, so I asked for Toffee to be checked over whilst she was there. She says he is how he should be 2 weeks after the operation, and took his temperature which was normal.
Obviously we are still keeping a careful eye on him in case it recurs. The vet suggested the wound had sealed but getting him to walk round on Sunday opened it again so it can continue to drain by itself. He is still a little swollen, so there is obviously still fluid leaking out.