I do agree that a larger tank would do them well, platys do not get very big but they do produce a large amount of bioload. I, personally, have a controversial opinion on stocking- if your fish cannot outgrow your tank and they have plenty of turning around space, and as long as you can take care of the water needs (whether the water changes is just once a week or more) then you should be okay. I use AQAdvisor Stocking Calculator to make the majority of my stocking decisions.
Here is what I have on platys:
Feeding:
You can easily overfeed platys and other livebearers as they tend to over eat. They do not have an efficient way to say they are full and being bloated can cause a lot of harm to their internal organs.
Only feed them once/twice a day depending on your water parameters.
Water Parameters for Livebearers:
Livebearers are hard water fish, needing upwards of 160-220ppm gH.
pH should be around 7.2-7.6.
Temp should stay around 74-82 degrees fahrenheit (23-28 celsius, as I see you are from England).
No ammonia or nitrites should be present, if they are, please do a large (75%) water change.
Nitrates should stay below 20ppm.
To test these things, purchase an API Liquid Testing Kit from your local pet store or online.
Tank Size:
Bigger is always better for livebearers, they don't necessarily require a load of space but they do produce a lot of bioload and if you cannot keep up with the water changes in a smaller tank, then you need to buy a bigger tank.
I recommend a minimum of 10g tank for the livebearers but most people will say 20g works best.
Hope this helps, I see everyone has given some input as well. If you have any questions, feel free to reply to any of us.