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More leverage and/or heat if you can apply some directly to the head of the bolt without melting the lower timing belt cover! 
Wingers wrote:if the engine is still in the car then get a long-ish ratchet or breaker bar on there and rest it against the floor so it can't move, then flick the starter motor and it will undo itself.
(I've done it plenty of times myself in the past) but some people have a fit over it 
£20 and ask him to fire up his compressor,......(by the clutch cover) and the top (by the starter) and got a 4 foot breaker bar, and scaffold pole, and it FINALLY came loose!"Snapped an Inch Drive Impact gun..." a week to get it off.
.. This actual one...(carefully) into the centre part way. After that it came away with a long diy breaker bar. Ranks up there with the CAT bolts after the turbo ](./images/smilies/eusa_wall.gif "Brick wall"){kind=small}
.. tried WD40 etc ... left it soaking for a few days and still no movement. Placed that on the pulley until it went hot and then used a rachet bar to unbolt the bolt. But, a heavy lump hammer also helps. has to be that way otherwise they come undone and your cambelt and pully will wonder past you as you break down 
, so I ended up welding another nut onto the top of the existing one got it off that way, unbelievable what people do. normal circumstances I firstly tap the bolt with a lump hammer then use a mega big breaker bar and it usually comes off with no hassle 