Hi Peter,
check out my experiences with exactly your predicament at:
http://www.fornobravo.com/forum/f21/...-4-a-2045.html (Neill’s Pompeii #4)
I found that you start with a 5:1 mix vermiculite: portland at the very top of the dome and work your way down and around. I also found that the first coat was the hardest and added approx 1/2 pt of fireclay which made it stickier.
I used a brickies trowel (as I didn't have much room to work with a hawk) to hold the mix and a steel cement trowel to scape it off and ALWAYS work your material UP! It will simply continue to fall off if you don't but you soon work that out. Don't try to put more than a 1" layer on at a time, again I found that it causes more trouble than it was worth. Apply 3 X 1" layers and that should be enough. My oven at 500°C inside is cold on the top outer dome.
Neill