Here's another point of view:
Ph test kit £5.99
Nitrate test kit £8.35
Nitrite test kit £8.35
Ammonia test kit £6.99
KH test kit £6.95
GH test kit £8.20
Iron test kit £8.35
Phosphate test kit £10.79
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Total = £63.97
Now, I can think of a lot better ways to spend £64. I mean, that will buy a lot of nice plants. It might even buy a CO2 regulator. One can read about nitrate, ammonia and nitrite (for free) without ever measuring them in the tank and know more about these parameters than what one will ever learn by testing for them. On top of all that the kits are notoriously inaccurate. A chimpanzee can be taught to test for nitrate but the chimp won't have a clue how to interpret the results of the test. This is what we turn ourselves into by immediately flocking to test kits. That folks associate test kits with growing plants is one of the biggest scams of the new century.
The sinister effect of test kits are based on two prime factors:
A) The user attempts to micromanage the values in the tank by attempting to keep in between a set of values.
B) The user believes the results of the test to be accurate and then takes action based on the faulty results.
It's a much better strategy to first learn the how and why of plant growth, to follow the procedures of filtration/cleanliness, flow/distribution & nutrition and to understand the principles of lighting. After being grounded in these principles one can then decide on the use of various test kits with eyes wide open. I'm always appalled when it's suggested to do the reverse because it only contributes to more confusion and more mistakes.
As far as I can see, if you already have a tank then presumably you are well grounded in the fundamentals of a tank and I see no reason whatsoever to add any more test kits than you have now. The decision of RO versus tap, as Mark suggests should only be based on the fish. The overwhelming majority of plants do fine in tap water as long as the nutrient levels are adequate. Start here=>
EI DOSING USING DRY SALTS to get a grip on the rudiments of plant nutrition. Then you can find alternative dosing methods using these principles on
JamesC Discusses Dosing Methods
I'm not familiar with your tank brand so I assume this is powered by 40 watts of T8? You can grow plants with this configuration but it's worthwhile investigating adding either CO2 or perhaps adding CO2 by use of Excel liquid which is much easier but could become expensive over the long run. If you elect to go the CO2 route then please review
Adding CO2 via Fire Extinguisher as well as
CO2 MEASUREMENT USING A DROP CHECKER
There are other articles in the Tutorials section of the forum to get you started. So as you can see, there are so many more important issues than what test kits to get it's ridiculous. Have a look at these articles and ask questions if anything is unclear.
Cheers,