Thanks Human.
UK electrical fittings have gone metric whereas I don't suppose US Kirby's or fitting will have.
a bit of research on the internet suggest that the screws you're describing as US anchor screws are 8/32
#8 screw with 32 threads to the inch
or maybe 6/32
UK light fittings use m3.5 to hold the plate to the wall but I'm not sure of the anchor screw spec. Think it used to be a UNF or BA thread but imagine it's gone metric nowadays. The trouble with Google now is you get flooded with all the rubbish answers and the info you need is hidden away.
I'll try experimenting with the other screws on the Kirby and if I can identify it fully, I'll let you know.