Put the cream and the lemon zest and juice into a saute pan and bring to the boil. Reduce by a third- it will reduce quickly.
Meanwhile, cook the pasta in plenty of boiling salted water until al dente, and cook the peas separately until just tender.
Add the peas, Parmesan and seasoning to the cream. Drain the pasta and add it to the pan along with the ham and the chervil. Heat everything through for about 20 seconds and serve immediately with extra grated parmesan.