I done another water change today and it is now acceptable. I dread to think what the reading was at first.
For info: The pond is 1200 litres, with fish and I'm now using a Blagdon Minipond 6000 filter with a Hozelock 2000 pump.
I was originally using one of the in pond all-in-one filters but it wasn't really up to the job. Constant cleaning. If I was upgrading an external filter, I would have swapped the bio media over, but obviously didn't have that option.