I'm due to start a PhD in October, and I know its going to be a lot of hard work. However, I've been checking out my new university's sports and social clubs and I would love to join a few. How do the PhD students out there find a balance between work and play? Would I be able to go off on hillwalking trips every weekend and play tennis a couple of nights a week without it having an effect on my research?

I wouldnt recommend joining several - maybe one but dont expect to have the time to play for the uni team. Your boss will expect you to concentrate on your work rather than playing sport.

Work-load varies, but you should be able to indulge a hobby a couple of nights a week. You might not be able to go hill walking every weekend (particularly when you get towards the end), but you should be able to get out most weekends.