Start by making the sweet chilli sauce: finely chop the chillies, keeping some seeds intact, then peel and finely chop the garlic.
Place the sugar and 1 cup of cold water in a medium saucepan over a high heat. Bring to the boil, then reduce the heat to a simmer. Add the chillies, garlic, a good pinch of salt and a good splash of vinegar, then cook over a low heat for 10 to 15 minutes, or until it coats the back of a spoon. For a smooth syrupy texture, whiz the sauce in a food processor if you like, or leave it finely chopped, then put to one side.
Place a deep-fat fryer or a large, deep saucepan over a medium–high heat (roughly 180ºC). Fill three-quarters full with vegetable oil and leave to heat up.
Meanwhile, wash and pat dry the squid. Slice the tubes horizontally into rings or slice and open them out, then halve lengthways and cut a crisscross pattern onto the surface. Place the squid onto a large plate, along with the prawns, making sure any black bits have been removed. Finely grate over the zest of the limes and squeeze over most of the juice, then toss to coat.
Place the flour and a good pinch of salt and pepper into a large bowl, then add the squid and prawns and toss to coat (you’ll need to do this in batches).
To test if the oil is hot enough, drop a piece of bread into the pan – if the bread floats to the surface, sizzles and turns golden, it’s about right. Using a slotted spoon, carefully lower the prawns and squid into the hot oil and fry for 3 to 4 minutes, or until golden and crisp on the outside and cooked through (you’ll need to do this in batches). Transfer the cooked prawns and squid to a double-layer of kitchen paper, then repeat with the remaining seafood.
When the last batch of seafood is almost done, squeeze a little lime juice over the parsley sprigs, then dip into the flour and lower into the oil for a few seconds – be careful as it’ll spit.
Serve the crispy squid and prawns straight away, with crispy lime-spiked parsley, lemon wedges for squeezing over and the sweet chilli sauce on the side for dipping. Keep any leftover chilli sauce in sterilised bottles and store for up to 2 months in a cool dry place.