Good points, Bill. But by "smallish amounts" as you copy/paste above, I meant of the actual money, rather than of the goods it could purchase. It seems that £10 was regarded as a reasonable tip (ie a 'smallish amount') for a young man to receive at the end of a visit to older relatives [for those who haven't read the book, he is a midshipman on leave who has been visiting the wealthy relations who have adopted his sister, and is returning to his ship]. But by the analogy of the change in land values cited in The Times which I quoted, that is equal theoretically to £1,300 in today's money; which nobody would regard as a 'smallish amount' of the sort which could be casually handed to a young visitor on his departure.
I take it there should be some assessable economic/historical explanation for this anomaly; but blowed if I can see quite what it might be.