Hi and welcome to the BBS and collector's corner (the best bit of the BBS by a country mile). The sight ramp is on a dove-tail iirc and you may have to apply a little gentle heat and ease it forward and off (carefully ).
See diagram in link as here: http://www.gunspares.co.uk/shopdispl...=24684&cat=Mk2. Someone will be along in a moment with a better solution maybe, but while in doubt, do nowt. Atb and once again welcome to the BBS : Gareth W-B.