You are not overstocked in terms of fish mass/water volume, but you do have some quite serious stocking issues that I will explain.
First, water parameters...as you are in the London UK area I assume you have fairly hard water. This will be OK with respect to the livebearers (mollies especially, guppies, swordtails) so that is not an issue. The other species are more soft-water oriented but not to the extent that it would be with wild caught soft water fish, so we can let this pass. The next issues are much more significant.
Temperature. This is crucial for fish because they are ectothermic and temperature of the water drives their metabolism. Just a couple degrees higher or lower than their preferred range can mean stress, a weakened immune system, and general weakening of the fish's internal physiological system. You have species that must have warmer water, namely the rams; all varieties of Mikrogeophagus ramirezi need a temperature than is no lower than 80F (27C) and preferably a degree or two above this. However, this will cause real problems for the Zebra danios (Brachydanio rerio) which being active swimmers will be healthier in cooler water (range 18-25C/65-77F) and the same applies to the Denison Barbs (Sahyadria denisonii) whose range is 15-25C/59-77F. Both of these fish will begin having problems if kept at the temperature the rams must have. A note on temperature ranges...with reliable sites the fish are best mid-range, within a few degrees depending; the upper and lower limits are temperatures the species should be able to tolerate but not permanently, only temporarily. So these two species need water in the low 70's/20-21C. The tetras (Gymnocorymbus ternetzi) also willnot work at the over-80F/27C end.
However, the three above species have other issues affecting them or affecting some of the other species mentioned.
Sahyadria denisonii attain six inches (15 cm). Generally peaceful, but this species should not be kept with smaller fish that will likely be seen as food. It's mandatory water flow and cooler temperatures limit suitable tankmates to medium-sized barbs and danios, loaches, Garra and Devario species. Must be kept in a group, minimum 8; reports of aggressive behaviour may be due to the fish not being maintained in a sizeable group. A 5 to 6-foot (150 - 180 cm) length tank is minimum.
Brachydanio rerio are active swimmers, and this will annoy to no end the sedate fish here (angelfish, gourami). They may also turn to fin nipping as they settle; a larger group might eliminate this aspect, but there is still the very real non-compatibility of active and sedate fish in the same tank.
Gymnocorymbus ternetzi is also a shoaling fish so a group of at least six, but this fish too is prone to fin nip sedate fish so a larger group for the nipping but it cannot remain with sedate fish regardless.
Angelfish. This is also a shoaling fish, and a group of at least five in a 4-foot tank might work. If these two are both males, one will be dead in time, guaranteed. If both female, hard to say. If male/female, also hard to say, because they must bond to be peaceful over time. Angelfish must select their mates or bonding frequently does not materialize. Actually this applies to many cichlids, including the rams. As time goes by, they can literally overnight decide enough is enough and one will be hounded to death in short order.
These are the most serious issues now present, and obviously some fish must be removed from this tank, depending which way you want to go. I'd be happy to answer any further questions as you sort through this.