I have replaced some of the cells, two on one occasion and one on another. Both times it kept the car going for a few months and then another cell would die. I suppose that if I replace one (or a small number of) cells now, then it will again keep the car going temporarily, until another cell dies. The ideal solution is to replace all.
On balancing, I measured the voltage of all the cells the last time that I had the pack out of the car, and I used a headlight bulb and wire to take energy out of the ones at the highest voltages. The problem I see though is that each "cell" that we are talking about is actually 6 individual NiMh cells in series, and those cells inside can be imbalanced too. And there is nothing I can do about that.
Attached is an image of a replacement cell from ebay.@toffee_pie I couldn't find this cell type on that website (though very similar ones are there)
Any further suggestions are much appreciated.