Back again! I've been looking things up and I am spoilt for choice as you've all got such nice ideas but I can't do them all.
Mrs G: I think pyracantha is a great idea, but can't face tackling the thorns. Scabious sounds great and I love bluey-purples.
Verdun: Wow, thank you for such detailed suggestions, most of which I didn't know at all. Heucheras are definitely in my plans now. And blue festuca and ophiopogon too. I do want some lavender, so perhaps if I put it right next to the path it wouldn't swamp the box. But I can only find normal ones, not dwarf.
Obelixx: I think you're right about the euonymus and maybe that's the answer for under the window. It's 3' to the sill, so that sounds like it might be the right sort of size and coverage I'm looking for, so long as I can keep it in check without it getting too bushy.
Forester2: You're spot on with the viburnum tinus! That's the only thing I've bought so far and put it in the corner next to my neighbour (after I took that photo). Glad that it's the right sort of thing, but I thought it might be too big to have under the window?
BobtheGardener: I love your rock and raked gravel idea, but I think it would be a full time job what with the local cats and foxes mucking up the gravel. Also I need to walk on the gravel bit to get at the plants. But maybe somewhere else in the future..
punkdoc: You're psychic - I've got an acer in a pot at the back of the house! It would look nice, but I know neighbours have had pots nicked from front gardens before so I'm a bit wary of putting it there.
ginglygangly: Thank you for very useful ideas and advice. I think you're right - it's tempting to have lots of different plants, but I need to be a bit selective.
I've been trying to find the plants mentioned, but local garden centres etc seem to have quite limited stock now. I've found most of them on the Telegraph garden shop online, but I've got no idea how good they are, and I feel a bit unsure about ordering lots from there and them being tiny plants or not very good quality. Has anyone got experience of using them? Or where else online is good to order from?
Sorry for all the questions, but now that I've got started it all seems so exciting and I can see how much there is to learn, but I want to get it right. I think I'm going to try and cut my choices down to 5 or 6 plants and then repeat them, so that it's not all too mad and busy. And stick to blue/white/purple as colours to try and co-ordinate it a bit. Please do come back and tell me what you think. Expecially if my ceanothus hedge plan under the window is just doomed to failure....(I just REALLY like ceanothus...). Alternatively I'm thinking to use euonymus.