Bresco do the white plastic screw retainers for this, original part number is 621535, there are 10 in all.
I might be able to find a few decent used ones if you get stuck!
Where are the other 2 then Steve? Have the four one side and one the other hence why I thought I was missing 3, grills are in bad condition so have some replacements all intact, so guessing the 2 others are the long screw ways which are not on the set I've removed?
Yes that's right, the odd 2 go in the long tubes at the outside ends of the grilles. Finding screws for these is a right pain, mostly they are missing, with the tubes broken off and often the bit of front inner panel they fit to is missing as well! Fortunately 4 a side is generally sufficient, so I regard the end ones as optional!