Thought I'd give an update on this. It's basically a Denso gear reduction starter adapted by PowerLite to fit classic cars that would normally have Lucas. Running the smaller motor at a higher speed via a reduction gearbox to restore the torque makes for a more efficient and much lighter unit.
Mine worked very well for about 5 years. Spun the engine over much faster on a cold day when the original Lucas would struggle.
It then gave problems engaging. Turn the key and the motor ran, but sometimes wouldn't turn the engine. After a few goes, would.
Being a pre-engage starter, the motor doesn't run until the pinion is fully extended by the solenoid. Suggesting it was a problem between the pinion and ring gear - but both looked OK. No more than slight tooth wear.
Sent it back to PowerLite. They made up a story about the engine or flywheel being modified or wrong in some way, stopping it engaging. All they offered was a new one at a tiny discount. About a tenner less than the Ebay price of 225 quid.
Eventually got some time to strip it down. Anyone who was an expert on these starters would have seen the problem without even getting a spanner out of the box. But I'm not an expert on Denso starters.
The pinion runs on a spline on the main shaft. It is spring loaded and retained by a washer one end, and a washer and snap ring at the other. The spring and the far end washer going inside the pinion. The far end washer was missing - no sign of the remains allowing the pinion to slide freely for getting on for 20mm rather than the 2mm or so allowed under spring pressure.
Anyone in the least bit familiar with this design would have seen the spring by simply pulling out the pulley by hand (the job the solenoid does) As it is normally inside the pinion, it shouldn't be visible.
WOSP (and others) adapt Denso units in the same way as PowerLite. But do sell spares. Contacted one of their retailers expecting to have to buy a new pinion (which comes with the fitting kit of washers, spring and snap ring) and they are very kindly sending me a fitting kit FOC.
Know who I'll be using and recommending in the future. ;-)