Plinth preparation
bare plinth
Drill bits: 15mm, 2mm
Scrap wood block
Fine sanding block
Synthetic wire wool

Paint stripper (non caustic, e.g. Nitromors)
Danish Oil
DIN socket (for external speed controller, if selected)

Time: 1 hour
Prise off blocks for motor board unless you plan to keep it internal.
Keep all parts safe!

controller hole dimensions
External speed controller socket hole (optional)
Drill 15mm hole centred 11mm from bottom of central flat, and 95mm in from inside RHS corner (as seen from rear).
Soft-clamp a block of wood to back of plinth while drilling to avoid splintering edge on breakthrough.
Insert the DIN socket and mark the holes for the locating screws.
Drill 2mm pilot holes and test the fitting.  Remove and retain for later.

Time: 4 hours
If it is in perfect condition, just clean it.  Otherwise, strip the plinth back to bare wood (hard work for black ash, unless you intend to respray it) and finish with three coats of Danish Oil.
Stain interior back and right edges to match colour of inner plinth and baseboard. Alternatively (as described in Plinth Assembly) cut some plastic card to fit.