How can I fix my Westwood SI300 ride on mower?

When I try to engage the cutting mechanism, the lever will not push down fully into the "engaged" position and when you try the cutting platform makes a dreadful screeching noise followed by blue smoke! It was fine when I put it away last year... I have tried to look underneath but can see nothing caught in the blades or belt system. I would be most grateful for any advice on what I can do myself or failing that a recommended mower repairer in the Essex area whose charges are fair. In desparation!