@Gibbo You Mollies and Platties don't need 25-26C, you could set the heater thermostat to kick in at ~18C and chances are it will rarely kick in unless it's a nippy cold winter spell, as the tank lighting and filtration will give off latent heat to the water.
Your substrate looks quite coarse in the photos, otherwise I'd suggest a group of 10+ Corydoras catfish, but they like digging and a coarse substrate can mess their barbels up.
Bit confused about your Stockport water not being soft, I was under the impression Manchester and Liverpool tap water is quite soft, the latter is at least partially supplied from around Lake Vernwy iirc. Southampton water is quite hard, kH and gH ~11-13 iirc, but the pH is ~7.8 iirc.
If your water is reasonably hard, then small rainbowfish species could be an option, such as Dward Neon. However, I expect your mid and upper levels to be filled with baby livebearers in the coming months, so I'd suggest looking for bottom dwelling fish!
Beware of getting "cute little babies" of species that grow up to require far more space than your current tank. I can't see your tank's dimensions, I'd guess it is a corner 180l tank, if so stick to species that aren't too active and grow to a max size of ~10cm.
A number of African riverine species are tolerant of a wide range of water hardness, in my ~280l tank of little/peaceful fish there's a group of...
~10 Synodontis nigriventris
1x Synodontis congica (very shy for a syno, the biggest fish in the tank but I've had to pull other syno species out because they would bully this timid catfish)
2x Microsynodontis sp.1 (last of 10+ group, very active when in big group)
~10 Pareutropius mandvillei (last of 25+ group, midwater diurnal social catfish that are similar to more common P. buffei)
2x Congo Tetra (last of 10+ group)
~5 African Banded Barb (last of 10+ group)
Plus a few non-Africans...
~10 Dwarf Chain Loaches
1x male Halfbeak (born in another of my tanks days after purchase and rescued before the adults ate him)
L204 Flash Plec
From my tank of bigger and more rowdy African fish, Synodontis schoutedeni could suit your tank, but I'd expect it to go baby livebearer hunting at night.
A single Kribensis Cichlid could work, but if you get a pair, do not get any other bottom dwelling fish as Kribs are excellent parents just like my old Steatocranus Cichlids... They will bludgeon fish that go anywhere near their babies.
Anyway, this has turned into a much bigger post than I intended... Research water chemistry needs, temp needs, whether a species is happy solo or needs a big social group of 10+, whether they are territorial etc.