It's been a few years since I registered with most of the temp agencies in London, so I've lost touch with most of them, but here's a list in case to help with jobhunting in case you're interested. Back then, I'd find temp agencies through TNT, Evening Standard and The Guardian; TNT's website doesn't seem to advertise them.

The ones I'd recommend are Action First, and ROC  for their standby service; and probably Grosvenor Bureau for temp work in arts agencies, though it's been years since I last used them.  

Agencies
** Action First - http://www.actionfirstplc.com/
- This is the best agency I've used for getting me work on a regular basis!
* Synergy - http://www.synergygroup.co.uk
- A lot of friends have used them...
* ROC - http://www.roc.co.uk/
- They have a Standby service in which you can turn up just before 9am, and they allocate last-minute work to those people waiting.
ProspectUs - http://www.prospect-us.co.uk/
Adecco - http://www.adecco.co.uk/Looking4Work/Looking4work.html
- Their offices often specialise, eg. the Victoria Street branch specialises in government-based work.

Agencies - general
ProTemp - http://www.protemp.co.uk
HotRecruit - http://www.hotrecruit.co.uk/
Crone Corkill - http://www.cronecorkill.co.uk/
Eden Brown - http://www.edenbrown.com/
- General consensus about them is that they get you work, but they're crap.
Secs In The City - http://www.secsinthecity.co.uk/
Lets Temp - http://www.letstemp.co.uk/
WebRecruit - http://www.webrecruit.co.uk/home/default.asp

Agencies - arts
** Grosvenor Bureau - http://www.grosvenorbureau.co.uk/
- First temp agency I used, got me work at BBC then Arts Council
Tate - http://www.tate.co.uk/ (I found them a bit irritating, but worked through them a few times, including at the Barbican; friends at the Arts Council used them.)
Attic Recruitment - http://atticrecruitment.co.uk/main4.swf
Bluefire Consulting - http://www.bluefireconsulting.co.uk/
Covent Garden Bureau - http://www.coventgardenbureau.co.uk/
- Horribly unmodernised & lazy when I signed up with them (both times!), hopefully they've improved!

Jobsites
TNT - http://www.tntmagazine.com/jobzone/
LondonJobs - http://www.londonjobs.co.uk/
Jobserve - www.jobserve.com

Lists/Directories
Arts agencies - http://www.agencycentral.co.uk/agencysearch/arts/countysearch/england/london.htm
Google - http://www.google.com/Top/Regional/Europe/United_Kingdom/Business_and_Economy/Employment/Agencies/