Mr Fizzion wrote:
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?

In a hard water area the cold inlet to the boiler's hot water heating
circuit should be softened, with an ion-exchange softener, as it will
scale up in no time if not.

I don't believe actually filling the sealed side of the boiler with soft
water makes any difference, I do. To have a separate un-softened feed to
top up the boiler with would be an extra pain to provide and would be
nigh on impossible on those boilers that have a built in filling loop.