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.
Having another go with my Clevedon Pier photograph. This time I used a thinner application of oil for the sky and less etching time. 
The sky has come out much better, but the shorter etch time wasn’t long enough to allow me to make a decent print. So, I’ll have another go, this time with a longer etch and a similar treatment for the sky.
Clevedon Pier – again from a photo – laserprint transferred onto aluminium with acetone then etched in copper sulfate and salt.
Raising the temperature in a water bath made the solution very active – slightly over etched this one – see the halo around the top of the pier.
I will have another go, etching for less time (2 instead of 3 minutes) with the oil resist for the clouds thinned out.
