Here (at last!) are some of those 17th September to Diary handwriting comparisons I promised you. Sorry they've have taken so long.
I'm not as certain with these as I am with the 22nd Nov postcard (more research needed), but I still think the similarity in the style of the handwriting andin regards certain words (The 'about,' 'loves' and 'nevers,' for example) is striking. Especially when we remember that the 17th Sept letter is supposed to be a fake and the diary's handwriting isn't suppossed to look like anything.
One thing you may notice about the 'time' from the 17th Sept letter is that it has the same, curious, flat-bottomed 'I' that the 'police' from the 22nd of Nov postcard has. The diary also contains similar flat-bottomed 'I's'.
Kind regards,
Tempus