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.
I've not used Telegraph online , but I am sure they will be okay (they have a brand to protect). I have used Crocus - they aren't cheap, but their products and service are excellent. Also Hayloft Plants are good. If you can get to a garden centre, I would recommend it - you might be able to pick up some bargain perennials at this time of year and it's always better to see the plants. I wouldn't bother buying perennials online now, better to wait till spring - not sure there will be much on offer and you are bound to be disappointed with what is supplied, as herbaceous perennials (eg heucheras) are dying down for winter now and look rather sorry for themselves . It's a good time to order and plant shrubs though.
Ceanothus might be allright where you want it - possibly wants a bit more sun though. I recommend Euonymus - I have both the silver and gold varieties growing in my gardens, both as tough as old boots. I think I prefer the silver one, purely because it is part of my more "designed" front garden. It is really coming into its own now that everything else is starting to die down. Some of the leaves look like someone has splattered white paint over them. But if you are really keen on Ceanothus give it a try! It might thrive. If it doesn't, don't grieve, just whip it out and try something else. Taking risks is all part of the joy of gardening!
Ceanothus would look lovely trained under your window as it is very pretty but as GinglyG says do bear in mind that it likes a warm sunny protected wall to grow against and can be damaaged by very cold icy snowy weather (I lost one that way) but, 'go for it' as you have set your mind on a blue plant.
As forester says Ceanothus can be tender, I lost mine in exposed site. Google Ceanothus Californian Lilac, a variegeted one, lovely plant not as robust.
Thanks again for the suggestions. I've got everything now. The plants I bought from Wisley are lovely but I am HUGELY disappointed in what turned up from the Telegraph garden shop. Some of the plants are very poor quality, and the rose I ordered is a really sorry lopsided wonky thing which turned up in a bashed collapsed box. They were completely unhelpful when I called them up and suggested I post it back to them at my expense to get a refund. It would have cost me more than that to post it to them, so I really cannot recommend ordering from them.
What a disappointment for you. That is the trouble if you haven't dealt with someone before - we all live and learn. I suppose you are now going to a garden centre to replace the poorly plants. I hope it turns out alright for you in the end.