Tomdip - many thanks for the extra information. 400C near the top is still very impressive and probably typical for tandoori cooking. If the thermometer reads 'hi' it means it's out of range, i.e. too hot. I looked at those thermometers on eBay about a month ago and I'm aware there are various models, only a few of which go up to 500C (officially). I'm impressed you got more than you paid for, in effect, if your 350 model will actually give readings up to well over 500C.
Your tandoor looks very good and I should think it will generate a lot of interest for your pub.