Got a "new" (21 year old) two button Seat key from the breakers for £1, ( with the advise, "it will never work.")
Take both keys apart, frequency is on a label beneath battery, both 434 MHz, hopes rise a little, circuit boards different though, sigh.
Used the two key method for mk4 golf, as on yubetou (sorry, lost the link), and..........................................
it worked, astonished & very happy. Swap the metal key which also has the immobiliser chip & in business.
Fortunately the manual switches in the drivers door module lock are still compus mentus which makes this method possible.
Oh happy day, now to battle the peugeot.