Thank you very much for your reply, Marc.
Sadly I have lost a further 2 fish out of my original 4 today

. It seems they got stuck in the bubbles at the back of my tank(from a rubber bubble tube attached to an air pump) & just keeled over on their backs & floated up to the surface dead. I am now left with a very lonely orange tailed Guppy, which looks healthy enough.
Luckily I only started with 4 fish I guess !.
Can you recommend any good testing kits on the market that test for Ammonia & Nitrates, as I tested the water for Ph, hard water & buffering zones before I bought the fish & the water seemed OK.