ammonia still at 0.25ppm but nitrites/nitrates both nil - isn't the ammonia supposed to turn into nitrates then nitrites? I'll do another 25% water change tomorrow and see what happens.
Ammonia -> nitrite -> nitrate
When you have enough bacteria and are doing weekly water changes, there should be no ammonia or nitrite but nitrate will build up. One of the reasons for doing water changes is to remove the nitrate.
With a fish-in cycle, you start with no bacteria. Ammonia is excreted by the fish and builds up, that's why you have to do water changes to keep it low. Slowly ammonia eating bacteria grow and they convert ammonia into nitrite, so as these bacteria grow nitrite starts to build up so you need to water changes to keep that low as well. Once there is nitrite, the nitrite eating bacteria can start to grow and they convert nitrite into nitrate.
It takes a long time for enough of both types of bacteria to grow, and the nitrite eaters can't even start until the ammonia eaters have made some nitrite for them to eat. It does sound wrong doing water changes to remove ammonia and nitrite but even a trace more than the current number of bacteria can eat will stimulate them to multiply. Eventually you will have just the right number of bacteria to eat the ammonia made by the fish and the nitrite made from that ammonia.
Re your tests:
0.25ppm ammonia could be a false reading as some people never see the yellow colour of zero. As long as that doesn't go over the 0.25 colour, I'd regard that as being zero.
Nitrite is easier to see the zero colour. With the API test if it is blue it's zero; if there's a hint of purple, it's not. With several fish in your tank if there weren't enough ammonia eaters I would expect to see your ammonia reading a lot higher than 0.25. Since you obviously have a fair number of them, they will be making nitrite. And again, if you didn't have enough nitrite eaters I would expect to see a reading for nitrite.
Nitrate is trickier. That should go up if you have both types of bacteria growing. And it is very unusual in the UK for there to be zero nitrate in tapwater. And you did report nitrate of 40 in the biorb in your earlier posts
Have you tested your tapwater for nitrate? Unless you have a heavily planted tank, you can't get the tank nitrate lower than the tap level by doing water changes with just plain tapwater (plants use nitrate as fertiliser which is why it can be lower if you have a lot of plants).
And just to check - you are shaking one of the reagent bottles, the instructions will say which one. All makes of tester have a reagent bottle with a chemical that settles out on the bottom. the shaking is necessary to redissolve it.
You have 7 platies, 5 male and 2 female? Unfortunately that is not a good combination. The recommendation is to have twice as many females as males as males will harass females and with more females each one gets time off while they chase another. Yours are probably quite young at the moment, but as Colin says once they reach puberty, things could change.