So found a long reach milling cutter tonight and though it a shame not to be able to use it.
First picture timed at 7:24pm
The generic Bligeport, like a Bridgy but on steroids.

Five tins of spinach later, head off, slotting head off ram off and 4" riser block fitted.
This pic timed at 9:24.

Now got a genuine 511 mm from spindle nose to the bed.

That's a 600 mm rule in the pic.
Sod it off down the pub, pie and a pint calls.