[Tilesathome] Missing lowzoom tiles at z6-z11

[Rob Reid]

I noticed this morning that some lowzoom tiles that I know were there 
last night appear to have vanished.

At zoom 6 its looks like about 5 tiles have gone from the north and west 
of New Zealand.

http://www.informationfreeway.org/?lat=-41.236175438892154&lon=173.87055867262404&zoom=6&layers=B000F000F

Is this the result of server based lowzoom stitch failing and removing 
the old tile or some other process?

I can see that there was at least one change in this area yesterday :
http://trac.openstreetmap.org/changeset/9602

On a related note, for the last few days I have been fixing most of the 
oceantile problems you can see around the coast of New Zealand and 
re-rendering and uploading the z12 tiles.

What prompts the server to do one of the nice new lowzoom stitched 
tileset at z6? Is it possible to manually request one? Following the 
instruction on the wiki I can render the z6 caption layer but an not 
sure how the automated server stitching kicks in.

Cheers

rcr