one of my employers did the same thing when i was going to start up on my own, he knew i would take a lot of his business so started to panic!
i read up on restrictive covenants and the best site i found was a solicitors website called
www.lindermyers.co.uk
they can't stop you using what you know to earn a living as it then becomes restriction of trade.
gardening leave means you are still employed by them but they don't want you on site for the reasons somebody above has already given but if they need you they can still call you back in to sort out the issue as you are still technically under contract