I've kind of fazed out after my Grade 6 exam, and I haven't been playing many new pieces for ages. Does anyone have any recommendations for books to buy? I'm looking for pieces of reasonable difficulty - i.e. not pieces that are too easy, but not ones that are too hard and would demotivate me before I begin. A kind of collection of different composers would be best. There's just too many books to choose from! Thanks

For general classical collections you could look at the ABRSM published "Keyboard Anthology" - book IV is around grade 6, and book V is around grade 7. Each book has a wide selection spanning the centuries from Baroque through to Romantic, and some Modern too. There's First Series, Second Series, and Third Series - and a book IV and book V for each series. Each book is around £4.95

I'm sure others will have other ideas - this was just off the top of my head, and just knowing they have a good cross-section of pieces across the years.

have a look here, for example

That's a good suggestion, and there's also the Romantic Sketchbook series (although spanning a period of only about 100 years) and other similar collections by AB.

Classics to Moderns (and More C to M) - you could have a look at books 5 and 6.