Put the saffron in a cup and pour over 3 tbsp just-boiled water, then set aside to infuse. Melt the butter in a frying pan over a medium heat, then cook the shallots, garlic and chilli on a low heat for 5-7 minutes, or until softened.
Add all the crab and most of the lemon zest to the frying pan. Allow to heat for a minute, then take off the heat.
Meanwhile, cook the linguine in a pan of boiling, salted water following pack instructions.
Drain, reserving a cupful of cooking water, then add the pasta to the frying pan. Toss everything together with a little pasta water, the lemon juice and saffron water with the strands. Season.
Divide between dishes and serve, sprinkling over the remaining lemon zest and a little black pepper.