To make the crumpets, mix everything together in a bowl and whisk until smooth. Cover and leave to stand somewhere warm for 15 minutes, or until bubbles begin to form on the surface.
Grease eight 9c/3½in tart rings with the butter and place in a large frying pan over a low heat. Fill the rings three-quarters full with the crumpet mix and cook for 10–12 minutes, or until the top has a skin on it and the mixture is almost cooked through. Carefully flip each crumpet and finish cooking for 3–4 minutes, until lightly golden-brown. Leave to cool on a wire rack.
Meanwhile, to make the potted shrimps, gently toast the cayenne, paprika, nutmeg and mace in a dry pan until fragrant, being careful they do not burn. Add the butter and lemon zest and cook until the butter has melted. Remove from the heat and set aside to infuse.
Shred the gherkins and kohlrabi on a mandolin, or coarse grater, into a bowl. Stir in the parsley, a drizzle of olive oil and a pinch of salt and pepper. Mix well and set aside.
Add the shrimps and lemon juice to the butter mixture and cook until hot and the shrimps are pink.
Preheat the grill to high. Toast the crumpets on each side until crispy and golden. Transfer to plates and spoon over the shrimps and the butter sauce. Top with the kohlrabi and gherkin mixture and serve.