do you need to dead head lychnis {gardeners world} thanks

Most flowers whether annual or perennial will flower longer or produce a second flush of flowers if you dead head them rather than let them set seed. So lychnis would be no different to anything else.
(Though to be honest, I never dead head ours, but then we do only get the one flush of flowers. Perhaps I should do as I advise!)

Lychnis coronarius is short lived (with me anyway) so dead heading would be a good way of keeping it going. I also cut it down when flowering is just over so that it developes a rosette of leaves at the bottom to keep it thru the next year.