To quote the installation manual for the Greenstar 40 HE plus:

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"In exceptionally hard water areas a device to prevent
scale formation may be fitted or, alternatively, the maxi-
mum temperature reset to about 45°C which may
reduce the risk of scale formation. The installation of a
scale inhibitor assembly should be in accordance with
the requirements of the local water company. Artificially
softened water must not be used to fill the central heat-
ing system. An isolating valve should be fitted to allow
for servicing. "
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According to Wessex Water, my water hardness is 280 ppm, which is
classified as hard.

Should I install a softener?

Is it a good idea to set the maximum temperature to 45C? (Presumably
this is the DHW temperature?) Would this result in any loss of
efficiency?

Thanks

Mr F