I sent a lengthy reply, which does not appear to have posted, however I short - many thanks, I will employ all of your suggestions.

Pool was shocked with cal-hypo 73%
SWCG appears to be working fine, will switch off whilst using bleach
Water drops 3-4 inches per week in summer, but a lot less in winter - pump running each time, suggests no major leak
Will buy test kit as suggested, thanks for links
Muriatic acid is back on the shelf to gather dust
Will increase brushing as much as possible - loooooong hours at work

Thanks for all the advice