Hi Lee,
Where in the uk is the market?
My family have been market traders for over 30 years mate i'm also from the uk, i spent most of my youth working on one market or another it was great fun.
The problem with these little indoor markets is they are normally small and tucked out of the way and get very little customers also the market trade has been going down hill for quite a few years now.
There's a reason they are offering you the stall for free probably because they can't get market traders to rent them, as they can't make any return on their investment?
Just look next time you go there and see how many empty stalls there are thats a good indication how good the market will be.
I do now this because i once had a lock up shop in an indoor market when i first left school selling womens clothes and god the trade was shit! and it wasn't the clothes because my family were doing 3 other markets at the same time and making good money.
If you are determind to do this i would look into some thing that people need on a regular basis, like fruit & veg or a food stall selling cheap biscits, chrisps, cakes, stuff like that and don't laugth because i now someone who makes a killing from doing just that.
If you got any questions just pm me.
Paul