I may have the solution? - I ordered some steel M6 female inserst and they arrived today ... about 20mm long and 10mm OD. A bit of lateral thinking and I remembered I had some knobs about 50mm across the plastic part - with an M8 male insert (they're from a Thule bike rack where I replaced the clamping knobs with locking ones).
Drilled out the knob's insert with a 10mm drill in the press and fixed in one of the steel M6 inserts with lots of epoxy - in the hole in the knob as well as round the 5mm or so of the steel insert that protrudes. Waiting for it to totally harden but an initial look appears promising.
What's less helpful is that I can't see how to take the plunge spring out of the router . . . it looks as if there's some sort of pin across the inside of the tube that's holding the spring .... but it doesn't move/come out. Surgery with a hammer and drift may be required but I'm going to see how the knob fares on the lift first.
If I'm not boring you I'll report back in due course!