Hey guys, thanks for your replies.

That Pixio PX-FD400 is exactly what I was looking for Tao. I can't seem to find one (or the Gary Fong) in the UK though. I'll keep trying.

If you get one spray let me know how it works. I have been looking at them for a while and I just haven't gotten around to ordering anything.

Tao, one of the smaller softboxes from Lumiquest should work fine, even on camera. I actually tried the Softbox III on my SB-400 the other day and it works just fine. Granted, I didn't have it on camera though, that would probably have been rather cramped. But, like I said, one of their smallest should work quite well.

This is the SB-400:

I like Lumiquest stuff, but I don't think that will fit as it is made for a "full" flash.

While searching for images I ran across these:
Pixio Flash Diffuser for Nikon SB-400 - Item Number: PX-FD400 Price: $5.99.
and
Gary Fong - Prizm Flash Diffuser (Canon 230/270ex / Nikon SB-400)

I'll second the lumiquest suggestion. They make several different sizes. If you're putting it on camera I would go for a smaller one than the softbox III.

B & H has the "Lumiquest Promax soft box iii." Just cut and paste into their search box. About $25 USD. It is 7.5" X 8.5" They may have others, but this is about the size which I have found gives the best diffusion on a small flash. I use it on an SB 800. Velcro is what I use to hold it on.

On that flash I just used a kleenex or tissue paper over it then added +1 flash comp.

I have never actually seen one for the SB-400 - it will be interesting to see if anyone has seen or used one. I had a friend fashion one out of a used rubbing alcohol bottle, that actually worked well.

Anybody know of a good conveniently portable little soft box for my SB-400 that I can carry easily in my gadget bag? I'd rather have a recommendation from yous guys than buy blind.

TIA.

EDIT: I prolly should have described it as a modifier as it is to fit on the camera rather than a stand? Anyway, I just want to soften my SB-400 on camera...