Took a sample of our water to an aquarium shop today & had it tested. Test for ph was 8.0
when ours still says 7.0 at home! His best guess was that there was something wrong with our test kit so I am going to look online for a new one (they are soooo expensive!). That means it must have been dodgy from the start & apart from a phosphate reading of 2.0 he was very happy with all the other results.
So relieved to find there isn't a problem after all!
Thanks to all for the support over the last few weeks - at least we now know that all was ok except for that one nitrite spike!