Thanks Carl.

Appreciate the advice. The pool meter that I have is meant to be quite accurate...
http://www.lovibondwater.com/product/scuba-ii.aspx

The Chlorine level reaches up to 6mg /l so we know that chlorine level is higher than this. I will try your method of diluting the water to get an proper chlorine reading.
I have read that my Cyunaric acid levels should be no more than 100 ppm, so although mine are high... they may not be exceptionally high.
Yes I have been adding some kind of baking soda to help stabilise ph.

A local pool person has suggested the following

- Keep the pool cover off to help the sunlight breakdown the high chlorine
- Brush the Algae so that it combines with the free chlorine in the water
- Add some fresh water where possible to help dilute
- Drain a little of the old water where possible to help dilute

He suggested doing the above before adding any more chemicals. Do you think this is a sensible approach?

Many thanks!