I would go for a zoo! frame for you 20"
Go to this link
http://www.tartybikes.co.uk/product.php?product_id=10878&category_id=36
Shop around for second hand deals around £179 maybe.
Not a bad frame for someone starting out.
Mine is not a trials one, but once when i was riding along a road on a bike ride we turned up onto the moors and i went off a rock that diddnt look that big at first, flew over the handlebars, landed in a pile of horse shite.
Today, I went for my first ever trials ride! Not the best idea in the ice! I have never ridden Trials in my entire life and within the day i learnt a 180 pivot and a stoppie rollback! I am so so pleased with my self. Is this a good start?, Do you think that that is any good? I'm currently running a Diamondback Kabuki.