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Hey Alan...
How good are you at scale drawing, mate? If I were you, I would draw the entire pier, maybe in an isometric style, at scale, using those reference pics, at the size that you are going to build the pier... Then eyeball the block size, unless you can get a physical measurement, then we could get a very close scale reduction...
~Gunny

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Hi Gunny,
The answer to that is hopeless. However, you've given me an idea. I've asked a friend to take some photos of the old pier in Bangor which is where my original idea came from, so while he's doing that he could measure a few blocks and I could use my newly found Scale Converter Site (Thanks Pat) to get a better idea.
Good thinking.
Cheers
Al