For the pickled pumpkin, halve the pumpkin, remove the seeds and then peel. Cut into thin slices approximately 3mm (1/8in) thick. Rinse the slices, pat dry, then sprinkle with the sea salt. Leave for at least 3 hours or overnight. Drain and rinse the pumpkin in cold water and then place in a heavy roasting tin.
Combine all the other ingredients in a saucepan and bring to a gentle simmer