Andy I don't know about finding original front sight elements but BBS member DM80 in Lancashire is more than competent to repair the Giss rifles.
I think BBS member UKNeil makes perspex elements that fit the 16mm(?) front sight on the Dianas with varying size rings on them (a bevel cut around the holes look like black rings when viewed through the diopter).
PS. the real experts on the match springers hang out in the Target section of the BBS, so you could try there.