Thanks everyone for your advice.
I think I'm going to give Newt_on_Swings' recommendation a try.
I'm going to empty the two accordion bottles of D76 into a bucket, then add 0.63L of water at 20 degrees Celcius, stir it, then pour it back into the accordion bottles.
None of my local sources of photo supplies have any developer in stock (and it took a couple of weeks to get the HCA), and I'm concerned that diluting 1:1 will give me coarser grain.
I'll report back with the results.