Are you in central Truro? I know of some at Mithian and others at Mount Hawke, neither sounds right for you - the buses to either are not that good.

I see that Transition Truro has some info -
http://transitiontruro.org.uk/index.php?name=News&file=article&sid=9

There's someone to contact (Richard) near the bottom of the page. I get the impression that there's a long waiting list.

Were the allotments you saw these?
http://www.crosslanesallotments.co.uk/

I hope you find somewhere you can get to.

Truro city council have 3-4 sites for allotments but the list was over 120 long when I joined three years ago and I'm down to approx #70!
I've been pushing for more sites as I have no garden and would like to be able to grow my own veg!
Richard is the contact for the parks/gardens but it doesn't get you far!
there are a few private sites just outside of truro, at greenbottom, bissoe/crosslanes and playing place.