What an interesting 'problem' Sue. We have two different varieties of Nandina, and one is only 2ft (I'll take a pic later); the others are 6ft but I don't THINK they'd mind being cut back. If purple leaves would be any good(?) I keep a Sambucus nigra down to waist height, but it has grown quite wide! Another thought for a contrast: Sorbaria Sem?

Have just put on a short blog to show you the small Nandina (Firepower) . . . the other Nandina is N. domestica and grows to 6ft (probably the one you were thinking of?). Good luck!

Hi, I have the Nandina Firepower, and I wish it would grow a bit taller, in a couple of Summers it has only grown to 2 ft. I know I am impatient by nature [I have told it off!] maybe it will grow a bit more next year. it's pretty though.

It's in very poor and dry soil and faces South BTW.

I've just looked up N.Firepower in a reference book, Freeasabird, and it says that 2ft is its standard height - sorry!

Cotinus coggygria has varieties which are purpley red, which go bright pinky red in autumn. They grow huge unpruned, but you can coppice them down to 6 inches or so every spring (after they've had a year to put roots down) to get larger leaves with brighter colour, and then they'll be 4 foot by summer's end.
Physocarpus has purple varieties, treat as above, or buy a dwarf form and prune out one stem in three to the ground each year.
Or for real drama, grow Ricinus communis from seed every spring, can get reddish bronze leaved varieties (I got mine from Jungle Seeds), gets to 5 or 6 feet by autumn and then dies. Toxic, so keep the kids away.
Or a red Berberis?

Berberis won't be too happy in dry soil - they tend to drop their leaves early. Pittosporum 'Tom Thumb' is purple rather than red, evergreen, and only gets to three feet.

Very interesting and helpful replies, thank you everybody.The Sambucus is a bit too dark for that corner and as you say would get too wide - I have one I keep about five foot six somewhere else and love it. The cotinus pruning is good news - I don't want it there but sounds ideal for a place I do want one but was afraid to venture in case it got too big. It needs to be where the sun shines through the leaves, which it would down on the South border. The lower part would be in permanent shadow from the conifer hedge - would that make the leaves revert to green?
Info on Physocarpus pruning is very useful, thank you. I didn't know you could prune them right back. The lighter red on might be just the job there. The larger Nandina might do either - just a bit worried about suckering as it will be awkward to get round to remove upstarts. I didn't know what the Ricinus was but see tit is the Castor Oil plant to which for historical reasons I have an antipathy and its time I got over it as its really very handsome!
I feel a lot clearer now - where would we be without all these wonderful knowledgeable friend to rally round?
Thanks again.

Pittosporum - yes well, it might be a bit exposed for it anyway - it was just the evergreen aspect that appealed. I have a small golden berberis in that border but put lots of gel crystals in the planting hole and it seems to survive withougt watering most of the time, but you're right to warn - I did lose a red fastigiate one in a similar position a few years ago because it was too dry.