Managed to get me a 3/4" BSP hexagonal nipple
from Screwfix.
Which allowed me to connect this:
To this:
I used to manually disconnect the DI hose when starting up the 4040 until the TDS settled down and then reconnect so the resin didn't take a hit.
But I've now finally got round to adding the valve set up, so no more disconnecting.
Converted all my 4040 fittings to 1/2" JG with tubing too while I was at it.