I solved it myself by replacing the useless soft plastic hooks with 25mm x 3mm aluminium strips bolted to the top of the grass box and bent to engage in the slots on the mower. I had the bits in my house and garage. Only discovered my question today while looking for a means of replacing the carbon brushes in the drive motor. They're not listed by spares firms I've looked at. Looks like my mower is scrap, maybe a blessing because it's been a pain to use for most of its life. Made in China, what do you expect.
Les F, September 2020