Have you thought about reclaimed setts. To cover the area you quote would probably be in the region of £700 (plus sand and cement)
Now that the issue of my whacky use of the gravel calculator has been resolved, the gravel is only going to cost £327 so I'm able to stick with my original plan. Reclaimed setts would look really great as a path through the gravel but that's definitely not something we could do ourselves.
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I live in west central Scotland - not where that photo is...
Gravel on paving can be very slippery - think Charlie Chaplin and marbles...