Thom,
I would guess your flash is plenty powerful enough for use at those short distances.
As for diffusion then if its a buy job I would suggest looking at a Stofen. Just dug up an old pic of my D100 with a flash attatched complete with a Stofen diffuser so you know what it looks like. For a diy job you could cut the bottom off a 1 litre milk container or similar and tape it to the flash.
Don
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