I didn't have a Phosphate test kit at the time of posting, but have just
come back from my LFS with a test kit which indicated .5mg/l of phosphate.
I also bought some Zeospeed which is apparently modified zeolite, to suck up
any excess phosphates.
I'm surprised at how low it was and was expecting a much higher reading,
though I have been fasting the fish for the past couple of days and I wonder
now whether I may have been overfeeding them all this time.
At any rate I'll take all of your suggestions on board and see what
develops.
Thanks everyone for your input.