Self-heal (proper name escapes me at present). Bees love it, grows anywhere, easy to come by and it is less than 12 inches tall. That's one for a start.

Can you not get some wild flower seeds.

You are spoilt for choice depending on use, soil and conditions and how much maintenance will be available?
Rockeries can work well on mounds like this? Phlox subulatas, Vinca major/minor, Hypericum - Rose of Sharon, Camomile, Cotoneaster horizontalis are a few. NP has already mentioned wild flowers - the Olympic stadium has planted wild flowers which were very popular. Also consider bulbs for spring, summer and autumn.

Depending on how much grass you have around the motte......how about a moorit sheep or two. Pets and efficient mowers of sloping grass. Arundel castle has them to keep the grass on their huge motte mown. My brother in law had a steep bank near his house to keep sorted and it nearly drove him potty trying to keep it free of thuggy weeds.

Trouble is Drc that most of the plants you mention are not 'native' to Britain and the brief was for native plants.
Thugs are always going to be a problem, unless you have really cleaned the soil and even then their seeds are going to be in the soil. So for a while you would definitely need to get in and remove dock and such like.
So, depending on aspect, Primula veris, Primula elatior, vulgaris.
If the soil is lime then Pulsatilla vulgaris.
How about Fritilaria meleagris?
Ajuga reptans
Scabiosa columbiara
Sedum acre, Sedum album. Sedum anglicum
Cirsium acaule
There is a few to be going on with.

:))

Thank you, all of you, for the replies so far. There are some gorgeous suggestions. I made a mistake with typing the dimensions and the "5" should read "15" - it has the footprint of a terraced house and is the height of a bungalow at its highest.
The trees removed were leylandii, Lawson's, yew, holly and elder and there was so little light for the last few decades that the soil is completely bare now those trees have gone - no weeds at all, so I should imagine that the soil is clean, i.e. there are no lurking docks, nettles, brambles, ground elder or other horrors just waiting for a bit of light, warmth and rain.
The surface represents quite a large area which is why anything planted would have to spread well and be largely self-sufficient as regards watering, weeding and clipping or cutting. I was wondering along the lines of clover as a good yardstick, though I don't know if clover would be appropriate for the site. I'm sure it would be lovely to have chamomile, but has any one seen the cost? !
I have uploaded a couple of photos to help picture the site.

Clover would work as a soil binding agent, as would a dwarf grass and still allow the flowering plants to grow through. Clover is a good bee plant too.
No matter what you end up with, there will be a need to strim the area in late September to remove any dead top growth and to spread any seeds around.

Actually had a senior moment then, I came on to say that of course there is always Wild Thyme. "I know a bank where the wild Thyme grows" sums up its preference perfickly.
It comes in different colours too from white to deep red.

How about heathers......?.

I'd go with the self heal if its dense ground cover you want, and dot some of the others amongst it. Ground ivy (Glechoma hederacea - had to look that up!)spreads like mad too and has pretty blue flowers. It makes a difference which way the bank faces too - if it faces north then native ferns would cover parts of it well. I'd only go for the fritillaria if the ground is fairly damp, but that would be just dot plants, not ground cover as the leaves are very slender and disappear in summer anyway.
If its acid soil heathers would do the job, If its on the neutral or alkaline side then scabious would be happy.
You don't say where you live - if near the coast in the south west, scurvy grass would do well. Scatter a bit of foxglove seed near the top of the bank and it should spread by self seeding. With wild plants a lot depends on the aspect and the soil.

What grows well locally is always a good guide