In reply to :
> Cheers, the exact part isn't mentioned in the manuals
You need the exploded view document: http://si.shimano.com/pdfs/ev/EV-BL-M675-3394.pdf
The diaphragm you speak of appears to be part of an assembly called the Separator Unit (as John Aisthorpe suggested). The part number is Y8VC98060 and SJS list it at £2.99: https://www.sjscycles.co.uk/brake-levers/shimano-deore-xt-blm785-separator-...
The interchangeability chart on that document indicates that the same part is used on the BL-M785 and BL-M640 brake levers. That may or may not be useful information (but see below).
The Shimano tech docs come in three main flavours for each product. The user manual which is prefixed UM, the dealer manual which is prefixed DM and often has useful information about setting up and fettling components that is not in the user manual, and the exploded view which is prefixed EX and which is where the part numbers are listed.
Another really useful set of documents is the compatibility charts: http://productinfo.shimano.com/#/com/3.7?cid=C-432&acid=C-432 which show you which bits work together. There is a set of charts for each model year so you sometimes have to go back in time a bit to find the bits you are looking to pair up with what - the archive link is at the top right of the page.
I can spend hours trawling through Shimano tech docs. I'm sad like that.
All that said, they're not infallible, so sometimes you still need to do a bit of well-directed Googling to flush out discussions on cycling forums about the particular component that you are looking to fix/replace/upgrade. Chances are that someone, somewhere, will already have found themselves in need of the same information as you, and sought help online.