With thanks to Jon' Lorton for this information:
The two main drive belts are listed in the manual as A30 and A31 - the former being the one at the front, driving the spindle, and the latter at the rear connected to the motor. Today, new belts may also be marked as metric i.e. the A31 as 13x760Li and the A30 as 13x790Li.
To change the belts first disconnect the power supply. The belt cover is best removed completely. Hinge it up from the rear into a vertical position and then lift it up and off. Take the opportunity to clean the pulleys and the area beneath the cover.

The two feed belts are listed as 2150 or Z370 and 2180 or Z445, both of which appear to be obsolete.   
However, a standard belt, the 10x425 Li (serrated on the inner face for flexibility) and made by Optibelt fits the lower clutch-to feed-shaft position.
As no suitable V-belt for the upper position spindle-pulley-to clutch can be found, a suitable alternative is an 8 mm Polycord belt that can be ordered here. A  length of 380 mm long to weld together is required (simple instructions are provided).

To fit the feed belts, first remove the main spindle-to-intermediate pulley (front) drive belt from the spindle pulley and also lift the main spindle pulley itself. Before removing the main spindle pulley, slacken the feed-belt tensioning roller brackets. For this job, a 3/16" A/F  hex key to partially unscrew the hex socket clamp screws is required. After slackening, pull the tensioning rollers clear of the feed belts.

To lift the main spindle pulley, you first need to slacken, or remove, the hex socket grub screw that's positioned in the boss below the belt/pulley housing, see picture below.

First fit the lower, clutch-to-feed-shaft, belt. To help getting the belt on, remove the cover strut which is held by a slotted head screw that is, unfortunately, not easy to get to. To fit the upper, spindle-to-clutch belt, first string it around the spindle pulley, underneath the largest pulley, then lower the spindle pulley onto the spline until the belt can be fitted around the upper clutch pulley.
Continue to lower the spindle pulley into its final position, taking care to get the flat on the spindle pulley bearing housing in line with the grub screw; then tighten the retaining grub screw. Fit the covers--and that's it.