[si-mate]

Right, I was completely wrong about the clutch operating on the idler. That is attached to a spring mounted on the chassis. The drive comes via a V belt (from the crank presumably) to a pulley mounted under the transmission.

I tried adjusting the cable at the handle but this had the effect of running the drive when the handle isn't depressed - which I guess isn't a good thing, so I've altered it back.

I put the roller onto a block, started the mower and engaged the drive. The rolled spins if there is no resistance, but when there is the belt and pulley attached to the transmission don't turn. I assume this must mean that the belt is slipping at the crank end?
I'm not sure how I can get to this to have a look. I was told that I won't get her to start again if I turn it over onto it's side.

[Gwiz]

hep, don't turn it over on it's side.
best way to do it, is on the top of a black and decker workmate. disconect the spark plug, and tip the machine backwards so that the spark plug is pointing skywards, and the top of the handles are now resting on the floor.
I'd put money on the belt either having slipped off the pulley on the crank, or it's worn to ruination and needs replacing. Knowing other Hondas, it'll be the belt off the pulley  
Keep me posted

[si-mate]

I've been off work and internet for over a week now as we've now moved house so here is the update for you:

I did exactly as you said and tipped her onto the handles on my workmate. I tied up the controls and turned the blade and nothing happened. I thought the gearbox was dead until I realised that I was turning the blade the wrong way!
Turning it the right way and holding the roller revealed the the belt was simply slipping on the crank. I took off the roller and there is an adjusting screw on the gearbox which tightens the belt.

She now pulls like a good'un!

Thanks for all your help!