Definitely a success - albeit a bit more liquidy that I wanted it, but it was creamy and very tasty.
I cooked it for an hour under foil without the cheese topping, then added the cheese and cooked it for another 30 mins with the foil lid off to brown the cheese... the cheese was a layer of Jarlsberg slices with grated mature cheddar on top; very decadent, but it worked nicely.
I'm glad I added some spinach into the layers as it really added to the dish, and the fish (basically just fish pie mix - smoked haddock, salmon and white fish chunks) were really good. Considering the amount of cream, butter and cheese in the dish, it looked and felt very healthy, and my Mum enjoyed it too.
There's also enough left over to use as a side for dinner tomorrow, so I'll have salmon fillets with this on the side.