The Rolls Royce B series engines use a Smiths x30065 thermostat but I don't know what the suffix is, so I can't say if it is the same as x30065/14 in your manual.
If the thermostats are the same, the Austin Champ Owners' Club replacement is:-
Quinton Hazel QTH222
Dia (mm) 58.0 57.0
Temp (C) 73 75
QTH222 is a modern style unit. The critical difference is the depth of the main flange that acts as the seal in the thermostat housing. The Champ unit is 4mm and QTH222 is only 1mm. If I were to use this stat I would cut a "washer" from 3.5mm cork gasket to compensate.
The above copied from here:-
http://www.austinchampownersclub.com/04 Club/02 Members Log In/01 Members Site/03 Parts/Off The Shelf Champ Spares V2.pdf
QTH222 thermostat:-
https://www.ebay.com/itm/Novo-C-75-Termostato-qth222-Serve-Para-504-505-Peugeot-Gasolina-1977-84-/111022060677