Laura
Just check the letting agent is legitamet (?spelling?!?) and is in your working area. How many lettings do they have i.e 500, 1,000, 35 etc. Do the maths and decide if it is worth doing - i.e how much is going to cost for the advert and how much work are you going to get out of it...
On a personal note, I went into something similar to this about 4 years ago, being the only cleaner in this brochure, guaranteed!! Distribution on behalf of the local council to about 60,000 tennants.....
NEVER SAW THE BROCHURE, NO-ONE SAW THE BROCHURE.....AND I NEVER SAW MY MONEY AGAIN!!!!! 
Besides, if you befriend a letting agent and they are really pleased with your work, they will send you all their work
FOR FREE!!I had 3 letting agents and recieved quite a lot of work from them on a regular basis, plus all their end of tennancy cleans, and because I painted a room once for them as a favour, ended up with a lot of decorating work too!!!
ALL FOR FREEAll the best with your decision, but take note of all the comments made in the previous posts before mine.!!
Regards
Tim