I understand that usually an airgun will undo the bottom crank pulley on a Focus 02 plate when changing a cam belt but is there another way if one hasn't got an airgun to hand?
I've seen crank pulley locking tools before but not sure there is one for my car?
Any help would be good please...

Put it in gear, get an assistant to hold the brakes hard (someone strong!) and then break it off with a 3ft breaker bar.

There might be a flywheel locking tool depending on which engine it is?

I am not telling you to try this, all I am saying is that I have done it this way on some cars (not the same as yours) . I put a big ring spanner on the bolt. I had the ring spanner pointing at about the 4 or 5 o'clock position if you imagine a clock face as you look at the bolt. I then put some bricks or blocks of wood etc between the end of ring spanner and the ground wedging it tight.  I then turned the key to activate the starter motor for a second - only the slightest amount. This loosened it.  There are some hazards. you don't want the spanner slipping off. You don't want the engine to actually start running (eg remove spark plug caps). if very tight maybe it might damage starter motor cog or ring gear. Like I say, not recommending it, only saying I have done it when desperate.  At your own risk!!!!

I have always been very wary of this loosening technique.

It's never obvious to me that setting it up like this will actually LOOSEN the bolt rather than make it tighter still.

ScaniaPBman.

I don't see how it could tighten?  I think all engines rotate clockwise these days don't they?  So if you get the spanner on the wrong side it'll just flick up and come flying out through the bonnet...

It's not a method I'd ever risk though.  Knowing my luck something would go horribly wrong!

this is how it went wrong for me once (I wasn't intending to undo crank bolt this time). I was changing a gearbox on escort mk5. To jiggle the gearbox shaft into the clutch I put a ring spanner on the crank pulley bolt to move it a bit.

Finished putting gearbox in and everything put away. Start the engine, I hear a big clunk, some sound of metal parts falling off and a split second later the engine starts.

WTF I think, open the bonnet, have a look and realised I had left the spanner on the crank bolt which had gone flying off, but before it did that it had caught on something and  it had totally removed the crank pulley bolt which fell on the ground. Luckily no damage done and I got it all back together ok. Luckily. 

Escort has pulley keyed to shaft, had that been a 1.6 petrol focus mk1 for example where the pulleys are not keyed to cranskshaft I would have ended up with bent valves.