Hi Yanyan
I am preparing for a Diploma but I havent completed my programme notes yet either!

I have started though, but like you, I am rather unsure what to put. So far, I have typed briefly about the composers history, when the piece was composed, when it was first published/and or performed etc.. Nothing too grand I know, theres plenty more work to be done!
I have a copy of Writing Programme Notes A Guide for Diploma candidates (available via the ABRSM website) and it states that at DipABRSM level programme notes should be written for a general concert audience, ie : non muscians who are interested in music and are fairly knowledgeable.
I too, would greatly appreciate any advice from anyone else currently preparing for the DipABRSM or anyone who has already taken it.