Steve, I used Izmir Tulum (just like cheddar) which worked perfectly. The last time I made it I also used it instead of the Parmesan in the mashed potato topping. I don't know how much Parmesan costs in the uk but Dogan has issues with what it costs here. The cream isn't necessary either and chopped spring onions in place of chives.
I simplified the recipe by only pre cooking the onions, the rest of the vegetables, broccoli, mushrooms, fish, herbs etc I just mixed up with olive oil. It cooks for 40 odd minutes in the oven and there's no need to precook.
I've made it a few times now and it's a really nice meal we both enjoy.