Hi Conor
I launched from Blackness Marine, just north of Dittisham last month. It's quite expensive but should be accessible at any tide state and has a pontoon adjacent to the slipway. The rigging area (by the slipway) is up a steep shingle track so you may need a powerful 4x4 to access it. Alternatively, speak with the office and ask if you can rig at the top of the hill. We then put my BRe on a visitor mooring by the Ferry Boat Inn in Dittisham and, because we were too lazy to inflate the tender, used the rather expensive water taxi (£3 ppew) to get us ashore. I think it runs from 0900-1800.
There is also Galmpton on the east bank but that is quite tidal. There are also many other options.
BTW - The lanes to Blackness are typical of South Devon, i.e. bleedin' narrow, so get some reversing practice in, particularly if arriving during rush hour. Don't try to access Blackness directly from Dittisham, as my reccie patrol (aka crew) said it was impossible.