Thank you for your post, Iain.
I looked up the Felco diamond sharpener when Danae sent me her post last night and I would agree, not fearfully expensive. I had no idea, however, what grit stones were, so I used google and this is what I found: What grit whetstone do I need?
Whetstones come in a range of grits.less than 1000 grit is typically used to repair knives with chipped edges.1000 to 3000 grit are used to sharpen dull knives.4000 to 8000 grit are finishing stones and are used to refine your knife edge.
Interesting. I never knew it could be so complicated; but, I'm sure, by the weekend I shall have the most efficient garden tools in the the neighbourhood, provided my local shops/garden centres have them!