I cannot believe how simple it was to follow your instructions. Within 30 minutes my oven was completed. I had to use an angle grinder rather than a saw - I see that you now recommend this. The other slight change I made was to make the top flowerpot fit outside the bottom. This way no sand or vermiculite can find its way inside the lower pot. This also had an unexpected advantage of providing a ridge half way down should you want to rest the kebab skewers slightly up from the heat.
Thank you very much for such an excellent idea. Various family members are rushing to Homebase as I write to get what are remarkably cheap bits of kit so they can make their own!
Thanks Guy for the feedback