Tip the carrot and celery and vinegar into a saucepan and place on the heat so the vinegar evaporates quickly. Splash in the white wine and reduce until practically dry. Add lactose free cream, bring to the boil, then simmer gently until reduced by two-thirds and very thick. Leave to cool. Cut the fish into bite-size chunks and set aside with the shrimp.
Tip spinach into a large colander and pour over boiling water from a kettle until all the spinach is wilted (you may need to do this more than once). Cool under the cold tap, then squeeze the spinach tightly to get rid of any excess liquid.
To make the topping, mash the saffron, garlic infused oil and chili together using a pestle and mortar. Tip into a saucepan with the lactose free milk, bring to the boil, then remove from the heat to infuse. Bring the potatoes to the boil and simmer for 15 mins until soft. Drain, then mash well with the infused milk, olive oil, lemon juice and zest.
Heat oven to 200C/fan 180C/gas 6. To assemble the pie, mix the fish and shrimp into the cold lactose free cream until completely coated. Tip the fish into the bottom of a large gratin dish and scatter over clumps of crabmeat. Unravel the spinach and lay it over the fish, then top with the mash. Cook the pie for about 30 mins until golden and starting to brown and bubble around the edges.