Others (on other threads) thought that the armrests can be levered/pulled off, and put back reasonably securely without removing the door card, so I tried it.
It can be done as follows, although I'm sure door card off is best for those willing/able to do it.
The rests need to be lifted up from the front edge. Once the lower tabs are free, the rest "turns out" releasing the upper tabs. For reinsertion, adjust the lower tabs slightly with pliers so that they match the lower slots, then match up the upper tabs with the upper slots, insert slightly, turn the rest down so that the lower tabs are close to the lower slots, check the upper slots agin, and tap the armrest home at right angles to the door (I just used the palm of my hand). The rests will then be quite firm, although much easier to pull off than they were.
Next time I have the door card off I'll sort the tabs from the back and get the fitting back to original strength.