I have been advised to wait before introducing some Neon Tetras because they prefer lower Ph levels. Does this mean that the Ph level will naturally lower on its own? I also fancied some Cardinal Tetras which seem even more particular.

Unless you are keeping wild caught fish you don't need to worry too much about the PH. If the water has a PH over 8.0 then add something to drop it down towards 7.0 but otherwise leave it be. The fish will adapt to the slightly higher PH and be fine.

The PH of the tank water will always drop over time. This is due to fish food and waste breaking down in the water. If you do regular water changes then the PH will remain stabile. A stabile PH is better for the fish.

If the tank has been running for a month or so and the filters are established, then you should be able to add cardinal and neon tetras without too many problems. Check the neons before you buy any as they often carry a bacterial infection that is highly contagious to other fishes. If there are any dead neons in the shop tank, or the fish have a pale blue line, or the blue line is missing in parts, then don't buy any.