From my recent French experience I know I'll make better use of a smaller, less-comprehensive, easily grabbed for dictionary for the moment rather than a heavy tome, but back then it was very hit and miss as to whether the translations/definitions were accurate, and reading around various reviews online it seems one or two of the older editions still have the same problem.
As a British English speaker the very US-friendly Katzner version likely isn't suitable; I've narrowed my choices down to the Langenscheidt Pocket Dictionary and the Collins GEM. I'm a little suspicious of Langenscheidt as they never seemed very reliable for German use, but its got rave reviews from native speakers and students alike. Collins always used to be my favourite, but GEM often have a limited vocabulary when trying to look up words from literature.
Help? Opinions on or experience of the two volumes I mentioned (or suggestions about an alternative) greatly appreciated.