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OK so done a few more experiments over the past couple of days.
Firstly I did another roast and you can see the profile here: http://imgur.com/...ry/X5Xiujv

I tried to keep the fan on high for as long as possible by covering most of the chimney, eventually had to reduce the fan and stir manually in the 2nd half of the roast.

Also measured the temps without any beans and the fan on full. Tops out at about 115 degrees C. If I almost completely cover the top it gets closer to 160 degrees.

Also did a few more measurements and measured 235V across the 41.5 Ohm heater coil. Although I cannot measure AC current directly, my calculations on the above suggest I am pushing over 5A through the heater and about 1300W. I feel like this should be enough heating power.

So I'm not too sure how to take it forward, in so far as I don't rally want to completely dismantle the whole thing to debug further (it was a right pain to get it back in the original housing).

Perhaps covering the top to create a roast chamber is simplest, not sure if covering completely would be a bad idea? Or maybe putting it a box so the hot air is fed back in.

Could also try larger batches and higher charge temp like 200 to see if that helps.