Unfortunately it's the self-tapping kind, they're plastic.

Look at the socket carefully, the one I have on desk has plastic threads that fit machine screens. If you had used self tapping screws before that thread would have been destroyed though.

Size wise just looked and yes M2.5 (3.5 seems to be light sockets) and total screw length is 35mm

OK - having actually removed a screw, then it would be smaller than that for lights & sockets. Mine are self tappers with an outside thread diameter of 3mm and length 35mm.

A quick look on eBay doesn't reveal a countersunk of quite that size, but a bit more persistence might find some.

The Solwise iPlate is still on their website at £2.94 inc VAT. Unfortunately it doesn't say that it includes screws, but looking at feedback on other sites selling the same product indicates it does, but I can't guarantee it. Note cost is £6.53 including carriage.

http://www.solwise.co.uk/adsl_splitters-faceplates.htm

Is the iPlate the same depth as the interstitial DSL filter?

Which also raises the question of what interstitial plate has he got,

Just ask an Openreach engineer when you spot a van parked up, I am sure they would oblige with a couple of the correct screws.
The new faceplates are just push on pull off but they have only just changed so I am sure an engineer will have a couple of the old screws knocking about.