Thanks guys for the useful replies. Just want to confirm - CH system is on hard unsoftened water - softener was bypassed to fill it - and bypassed if ever I need to top up. It is the hot water supply that is in question - the softener is on the mains side of the water supply so hot water supply...
Just had new Grant 26e combi installed (by same plumber who installed Aquadial water softener). Central heating system filled by unsoftened water as normal but all other water, including water going into boiler for hot water goes through the softener - I thought nothing of it as trusted plumber...