I thought all the TDS1 coolant pumps were made by Hoover ( of the carpet cleaner ) and used a Hoover drive belt. I ccan't remember if it was a Hoover Junior or Senior belt, and as AndyB says they were round belts.
I suspect your lathe has possibly been modified.
I've checked the Denford modification notes - there was a change in 1972 to the motor pulley drawing (ML100/201) "To make 'U' groove in pulley to a 'V' groove. To suit new type of belt"
(There had been a previous change to increase the diameter of the 'U' groove in the motor pulley to make the pump run faster).
I can't find an arrangement or parts list yet that shows the new belt or a different pump though.