The crab cakes can be made up to a day ahead – just chill after step 4, then continue with the recipe as above the next day.
Boil the potatoes in a large pan of salted water for about 15 mins, drain, then return to the pan and leave to steam dry for about 5 mins. Mash, then leave to cool.
In a large bowl mix the crabmeat, capers, spring onions, lemon zest and half the juice with half the dill. Stir in the mashed potato with some seasoning, then shape into 12 neat round patties. Transfer these to a plate and put in the fridge for 20 mins to firm up.
To make the dill mayonnaise, mix the mayo with the remaining lemon juice and dill. Put this in the fridge for later.
Put the flour, egg and breadcrumbs on 3 separate plates. Dust the crab cakes all over with the flour, then dip into the egg and finally coat with breadcrumbs.
Add enough sunflower oil to a shallow frying pan to come about 1cm up the side. Heat the oil, then carefully slide the crab cakes in; you might have to do this in batches. Cook for about 3 mins on each side until they are crisp and golden. Drain on kitchen paper. At this point you can pop the crab cakes into the oven to keep warm – or serve right away with dill mayonnaise and extra lemon wedges.