I checked under the plate by the fire valve sensor and that is perfectly dry. I also checked the flow rate and it is 7.2 cc / min which is exactly right for our model, the plate on the Aga says 7cc. I have reseated yet again the boiling plate and it seems to sit well on top of the barrel, I have also smelt around there when I have lite it and there is no sign of any smell escaping. The flame is burning blue and the shells are red, although rather than a cone of blue flame it is 2 separate concentric rings corresponding to the 2 wicks. The thing that puzzles me is it was working fine until it switched itself off at the end of June when the fire valve tripped and why does it take at least 1 hour after being lite for the smell to get into the simmering oven? I am completely lost as to what else I can check or do now. Thanks for all the comments.
This is the burner after about 1hr 20min of being lite, I have put a small video of the burner on Youtube:http://www.youtube.com/watch?v=IlhWyySBZME
I have also checked all the flue ways inside the Aga and they are clear.