Ceanothus seem to do this. You eventually get tempted to start pruning off dead bits but it gets worse and worse!! Take some cuttings from the new growth and, once they are rooted you can grow them on as replacements for the day when the Ceanothus finally turns up it's toes!

Yes, our ceanothus have done this too, but we have just left them, and one is back almost to nomal [to our amazement] and the other is just starting to fill out, so we are leaving them until next year to get a better picture of what is happening. It may mean a replacement plant [ we don't want to wait for years though for it to reach 7/8feet] so are not being too hasty.
Of course, if we have yet another killer Winter like the last 2, it will probably see them off.

Thanks volunteer and freeasadird, I would hate to lose it, it's quite a large space to fill

I can sympathise, we have a v similar one Taz. They are such lovely flowering plants, and the bees are BIG fans which makes it even better. They do seem to be a bit short-lived though and/or susceptible. I think ours is on it's last year but kindly throwing out some blossom all the same - as someone said here, it's a big plant to replace.