Brick wall pier spacing

I am getting my local brickie to build a garden wall using common 'heather' bricks. This will be a single skin width (i.e. 4ins) so will be stretcher bond, and will be 2.1 metre high. At approx 4 metre intervals there will be 450mm square piers which will be visible from outside (and inside). Obviously I will need some intermediate smaller piers (visible from inside only) to give some extra support. We have some similar ones on an existing wall which are 2 metres apart and project inside by the length of 1 brick (i.e. 215mm). Will this be sufficient, or will I need piers which are closer together and/or have a larger cross section? I am aware that there are guidelines issued to architects which give information on heights and spans for different parts of the country (depending on the prevailing wind strength) but I can't seem to find them. I live in the Reading area, so not too windy here! I could avoid the problem if I specified a double skin wall (i.e. 215mm wide) but this would be a lot more expensive.
TIA Bob
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'heather'
be
closer
spans
not
wall
When we did it I found information through Google, can't remember what words I used but piers and brick walls would be a start :-)
Mary

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