Thanks for the suggestions everyone, I have bought some additional nitrogenous test kits and will have a go at those tomorrow morning.
I did buy a few new fish recently ( now all deceased) so maybe they brought something in with them, although I lost a couple of kuhlies before they arrived. If the water has acceptable levels of other contaminants, I will keep monitoring and lay off of buying anymore fish for a while.
I have already checked the water I was using to replace the old water, it was approx 0-1ppm Phosphate
Will keep you posted on my progress, thanks again for the advice