Hi Parrot.   Thank you for your input, some of which I understood!

I have been told by Apple themselves, and by a person who works independently but specialises in Mac stuff, that I could go as far as Snow Leopard, but not further.  I don't really fully understand why (the technical specs are so much double-dutch to me) ... but I have accepted their diagnosis.

I completely see it is a numbers thing.  I think the stats are less than 15% of Mac users are still on Leopard or earlier.  Now, clearly 15% of Windows users would be a huge number, but the same proportion of Mac users is a pretty small base, so I take their point that putting the money in is not viable.

I have been working on the "exisiing edge" principle since FF stopped support at the beginning of the yr.  I think any "risk" is indeed small.  I'm going to upgrade to an iPad in order to do certain things, but will keep my Macbook for others where, should a problem occur, the inconvenience would be minimal.