I've roughly speaking finished. I made a sand form using fine beach sand. Covered this with wet newspaper, then piled on the clay/sharp sand mix. I used a 2 clay : 1 sharp sand mix. The sand was a mix of what's called 'CS' and Otaki sands from Placemakers. Basically a sharp sand mix with lots of particles going from less than 1mm through to about 3mm.
Also, check the firewood to ensure it's really dry. I thought the door must still be too small as it appeared to be smoking more than I expected so I filed some more of it away. Only then did I realise the firewood I was not completely dry.
And another lesson: I tried to use a hydrated lime/clay mix to protect the external surface of the oven. My initial attempt has resulted in something far more concrete like than I was expecting. It has deeply cracked during the drying process and left me realising that lime should probably just be applied as a wash. I think I'll have to scrape this stuff off and start again. Check the picture:
Finally, many thanks to Kei Takashima for his blog and for giving me the impetus to get up and give this a go - cheers Kei