Black specimens with red front
This group includes Red-rumped Ladybird and the males of four other Scymnus species:
Oak, Heath, Schmidt's and Dusky Ladybirds.
The wing cases and rear of the pronotum are mostly black on each species.
The legs, face and part of the front of the pronotum are orangey brown (red).
I have not yet seen red fronted male forms of Heath or Dusky Ladybirds but have seen several all dark female forms of each, suggesting that the all dark forms are commoner or easier to locate.
It is possible that some males of these three species have an all dark colour form, resembling females.
Red-rumped Ladybird
This is a distinctive species, as the rear edge of the each wing case is infused with a browny-red patch. The patch can be bright and obvious or dull and tricky to see.
The red front on the pronotum varies from a slim line (female) similar to Heath Ladybird, to an extensive patch (male) similar to Dusky Ladybird.
It is an elongate and relatively large species.
This is an elusive species of wet habitats, although it also spreads out into drier areas and can be locally abundant.
Best searched for with a sweep net.

Reddish rear wing cases

Typical male pattern: Extensive red front

Typical female pattern: Narrow red front
Oak Ladybird can also show a faint reddish tip to the wing cases but is a very different shape and occurs in different habitat (see below).
Oak Ladybird
This is a small and rounded species, with a short oval shape and highly domed "shoulder" area.
The wing cases are all dark but the red abdomen tip is usually visible extending beyond the wing cases. This feature is clearly visible if the underside is examined.
The amount of red on the pronotum front varies.
Strongly associated with oak trees and best found with a beating tray.

Visible red abdomen tip

Red abdomen tip

Extensive red front

Narrow red front
Heath Ladybird
All dark with an orangey-red face and narrow leading edge to the pronotum. This combination is shared with the majority of Schmidt's Ladybirds, (a few Schmidt's can have two or four red spots, though this is more likely in continental specimens).
Heath Ladybird lacks a groove along the centre of the metasternum (on the underside between the middle and rear legs) and the disorganised lines of punctures on the wing cases shown by Schmidt's.
A species of grass and heathland vegetation on dry sites, including sandy and chalky soils.
Best searched for with a sweep net.

Typical male markings

No groove on metasternum
Schmidt's Ladybird
The majority are unspotted, so resemble Heath Ladybird, all dark with an orangy-red face and narrow leading edge to the pronotum, (section behind the head).
The diagnostic feature is a groove along the centre of the metasternum.
Another feature is the presence of strong punctures on the wingcases forming lines resembling striae.
It is a long oval shape, similar to Angle-spot Ladybird.
This is a grassland species associated with moss and is very elusive.

Narrow orange front to pronotum not reaching rear corners. Orange face and legs. Disorganised lines of punctures visible on front of right wing case.

Groove on metasternum
Dusky Ladybird
This species is similar to Heath and Schmidt's Ladybird, except that the orangey-red colour on the pronotum is more extensive. On many, most of the pronotum is red with a small black patch of black, a semi-circle or triangle, on the rear.
There is no groove along the centre of the metasternum.
A short oval shape, rather rounded, similar to Red-flanked. Schmidt's and Heath are both likely to be a slightly longer shape but the differences are subtle.
This under recorded species occurs in the London area, and seems to favour dry vegetation in man-made environments.
An individual showing the above features and found in the right habitat, is tickable as far as I am concerned but would possibly not be considered a reliable record without a verified specimen.

Typical male markings

No groove on metasternum
In the Ladybird Atlas (2011), the four photographs in the Heath Ladybird section actually show Dusky Ladybird instead.
Some Red-rumped Ladybirds show a similar pronotum pattern and may have reduced red wing case tips, so could be a cause of confusion.
Red-rumped is a more elongate species and the reddish tips should be visible on closer inspection.
Oak Ladybird could also cause confusion but should show a reddish tip to the rear of the abdomen underside.