Finally, after five attempts, I’ve got a plate with;
enough etch to make a relief print,
a satisfying sky effect
enough detail in the ironwork (not etched away)
3.5 minutes in fresh, warm (hot tap) etching solution seems to be the effective recipe.
Using the edge of a feather to apply the oil has given a controlled application.
Using two-colour combination on the ink roller seems effective too. Now I have a plate that works I can experiment on the inking.