Hi there - sorry - been outside.
Shedding of the scutes in turtles is normal, as long is the shell looks good and hard and healthy underneath - not soft and spongey.
When you say that there is a heater, do you just mean a water heater, or do they have a heat lamp over the basking area too, as that's probably more important! They can cope just fine with room temp water temps (70f ish) as long as they can thermoregulate adequately out of the water.
Ideally you want the water a bit warmer than 70f, but its not essential, by any means (with the exception of if they're on meds that are generally metabolised better at higher temps., when you'd want to keep the water warmer than you otherwise would...)
The filter you already have is of the same type that I've found inadequate for a few large goldfish, when we used to keep them, nevermind turtles, I'm afraid. :(
This filter is the only one on the PAH website that would be worth having, and you'd need the 405 one, not the 105 that that landing page displays first... www.petsathome.com/shop/external-filter-by-fluval-25650
It's not necessary to have gravel on the bottom of a turtle tank. If it's the tiny bits of fish tank gravel I'd definitely get rid - they could ingest it and that can cause impaction. If the stones are bigger so that they can't be swallowed, then that's much better (nice smooth "river rock" type pebbles are preferable) but it just makes it far harder to keep the tank clean... They do look better than a bare tank bottom though, for sure!
The platform you linked to is very small. Probably ok at their age, but useless for adults. Not sure if you can get them in the UK (will Google later tonight) but this is the ideal sort of basking area (in large if for adults.)
Lights should be off at night but air/water temp not allowed to drop too cold.
Got to go into town again now but will check back later :)