Just on the off chance that there are other Poole brewers out there:

Please find below the average levels for the substances you've
requested.

Calcium (Ca): 112 mg/l
Magnesium (Mg): 3.5 mg/l
Sodium (Na): 12.8 mg/l
Sulphate (SO4): 16.8 mg/l
Chloride (Cl):26 mg/l

Alkalinity as CaCO3: 237 mg/l

This was from Wessex Water who replied to my question the next day! Contacted them on info@wessexwater.co.uk

You can use this info with any Water Treatment calculator to work out what treatments are needed.