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Which way to undo the bolt? I always get confused by this...

It will be a standard RH thread (so anti-clockwise).
[righty tighty; lefty loosie ] laugh

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Should they be even, i.e. all 6" give the fronts are (I assume) original, or is it normal for mowers to have a different diameter front to rear? I think there may be a slight tilt up front-to-rear.
All but a few mowers used same size wheels in the 1950s.
A couple of companies were bold, realizing the advantages
of large rear wheels. Carter and Turner and Supa-swift had
larger rear wheels in the late fifties. Other would follow,
and larger wheels would be used on more expensive models.

On the Eric chassis I do believe the wheels were all the same diameter.
Your front wheels are genuine Turner 'Delrin' (Du Pont) wheels.
The rear ones are not - in my view. Here is a photo of your
chassis on a Turner at the Royal Perth Show, 1961.

[Linked Image]

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Why are replacement wheels so expensive? I'll be up for $80+ for 4 plastic new ones.
I think the whole spare parts industry is controlled by big wheelers,
and, today, it is cheaper to replace a whole mower than buy one or
two key parts. However, Turner wheels do appear on eBay from time to time.

Best of luck
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Jack