Never worry about asking a question that may have already been asked it always differs from person to person at the end of the day
Welcome to the forum though! Sounds like you have had a bit of a pickle with your tank, first thing to say though is stop listening to your LFS because if the guy is telling you that platies cant handle UK water its wrong in the wild they come from Central America which has similar water to the UK at times, then again most store bought platies are farm bred so bear little resemblence to their wild needs you might just have had bad stock as these fish are often bred for quantity not quality.
What kind of filter are you running on your tank and how often do you do a water change and clean the tank? What size is the tank over all like in liters? When you cleaned out the tank in the overhaul how much did you clean the filter?
When the other fish died did they look sick? Bloated, lots of white spots, white patches on body, fluff around mouths or on fins, red streaking, thin and not eating, or scales popping out? Is it possible that one of the other fish could have killed them you sometimes hear of quite aggressive mollies could they have done it?
When you say you have 2 FMs in a smaller tank than 40 inches how big is this small tank? I would usually recomend a 40 inch tank minimum for 2 FMs so a bit concerned about how small this is other tank is. You may have had a problem free 7 months with them so far but pretty soon they will hit sexual maturity and if you have 2 males could be problems if you have a male and a female they might take over the whole 40 inch to them selves if its 2 females might be slightly less problematic.
Depending on the size of the other tank and once we have worked out whats going on in your big tank I would swap your stock a fair bit. Totally depending on the capacity of your 40 inch tank and the size of your smaller tank I would speculatively suggest something like -
In the 40 inch
And in the smaller tank depending on size
6 Rummy Nose
Depending on the other dimensions of the tanks and the capacity you may be able to add more fish to each tank.
I know its a lot of questions but its just the best way for us to get an idea of whats going on in your tank