Well I am making slow progress on this. Visits to garden centres are of limited value, in line with my limited knowledge. The only place I've bought anything is B&Q, where I bought some euonymus and some Buxus Simperverens. Nowhere had Hellebore. I've got about 9 plants and was going to plant that down the narrow part of the border up to the bush. Beyond the bush where it widens I was thinking of some ornamental grasses and some kind of bush or dwarf tree/bush. Haven't decided on the corner bit yet. All the Hebe I looked at was quite small and I need something to fill it now.
Crackle, I'm a nurseryman and just found this thread, this may be of help, though despite my profession Mrs Paul does the gardening.
For your tree I think a Japanese maple would be excellent but do grow it in a pot. Several things here, they can be tricky to grow in the ground and the spot you have will be tricky, no doubt about it. Secondly if the plant is unhappy you can move it and in a harsh winter like last you can more easily protect it - I lost two well-established maples due to cold. Your problem could be theft but only you know your area.
You will be unlikely to find any decent Hellebores at this time of year, wait till February / March of next. Any you do find are likely to be left over from an early spring delivery. I would avoid the orientalis types in this position as they are very leafy. Try to find Helleborus argutifolius (AKA corsicus), or one of it's hybrids for fancy coloured flowers / foliage, as this is more compact, has very attractive foliage and holds the old flower in an almost "dried" state for a long time. There is wide variation amongst this group of plants and you should visit a number of places to find one you like. You could also consider Helleborus sternii.
I wouldn't think of trying to use any artifical root barriers in the soil. A pot is a good barrier which you can always enlarge as the plant grows.
An alternative in the area, by the fence, which hasn't been mentioned is bulbs. You can go right through the season with a succession of flowering, dwarf bulbs, of every shade and flower type you can imagine. Snowdrops, blue bells, dward narcissi, dwarf ornamental allium, dwarf iris, anenome, fritallaria, winter aconite, crocus, colchicum, cyclamen. Easy to maintain, cheap after the initial purchase.
With regard to B&Q, they use some of the best growers in the country and have very stringent quality control at the point of delivery - into an RDC for onward distribution to store. IF you buy stock which has been very recently delivered it should be OK but remember these plants have been grown to a tight specification, with low margin and to hit certain retail price points. The quality issues with B&Q stock arise from a lack of instore care. As you appear to only need a small number of plants I would go to a quality garden centre and pay the extra for garden centre quality stock.
There are plenty of very good GCs in the north-west, it's just a question of knowing which one. You're on the Wirral? I haven't been in a while but Gordale, Burleydam and Grosvenor all have good reputations. For advice I suspect Burleydam may be the best choice as I recall they try to have a small and friendly image. You also have Ladygreen, near Formby, which used to be excellent but has been taken over and I don't know how it is these days. used to be reknowned for plant selection.
Good Luck!
PS i you chose bulbs buy quality. Cheaper bulbs may take a while to mature, potentially rot in the ground f too young and perhaps not flower the first year.