Well, after much trial and error (having lost all 3 of my guppies), I now seem to have perfected my tank conditions!
The pH is now about 7.4 - which is normal (I think!).
I added Stress Coat when I added the new fish too.
I've now got zebra and leopard danios, a corydora, albina glowlight tetras and swordfish in with my Siamese Fighter, neon tetras and bristle nose (algae eater).
They seem, so far, to be good with each-other, and the tank looks good!