I'm trying, and failing, to find bass clarinet music that looks interesting but playable! I'm Grade 7/8 on my soprano Bb, but I've only had my bass a year or so. I want to find some music that I will enjoy playing, and is more challenging than beginner clarinet books, but is manageable for someone of my standard. Sort of intermediate/grade 5 level. Any suggestions os individual pieces/composers/sites?

Look at <music-scores.com>

I guess you are in UK. The best source here is June Emerson Wind Music - www.juneemerson.co.uk

If you want to play with others you can always do some brainwork & play cello or bassoon pieces transposing direct from bass clef.

A useful thing to do anyway is to learn reading from bass clef as there are some orchestral parts writtten in bass clef for Bb & A instruments. Those sort the men from the boys!