Well, it's just had another 3 weeks with 500W halogen to dry, thanks to not getting time to work on it... No more cracks, so good news.
I filled the gap between the back of the chimney and the dome with vermiculite concrete - notice that the centre of the oven dome has got pretty hot with the halogen light and has gone very pale (no cracks, but far too hot to touch!).

I then filled all of the gaps round the oven entry arch with an insulating mix (my own recipe: 5 parts vermiculite, 2 parts fireclay, 1/2 part Portland, 1 part sodium silicate). This was a very sticky mix that set hard within a couple of hours.

This mix is the flecked gray stuff you can see in the picture above. This was great stuff for filling all those awkward gaps - just mould a ball of it and shove it in.

I also used this mix to smooth off the transition between the rectangular brick chimney and the round chimney flue:

This gets rid of lots of sharp edges and corners - hopefully the smoke will want to go upwards even more now...
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