It's a Handset No:1 that takes a Mic, Inset, No:13, same as on an Australian Type 400 instrument. The Handset No:3, which also used to use an inset 13 mic, incidentally, years ago, is a lightweight plastic (diakon on older teles) handset, not the heavy Bakelite thing on the GEC1000. The shapes are similar and both handsets use a rocking armature receiver.
The Type 13 inset mics are common enough and there'll be plenty kicking around.
http://www.britishtelephones.com/hands1.htm