decrepit said..
My shower taps are like this, I don't like wasting water so I adjust hot and cold for a fairly low flow. But after a few minutes, the cold flow reduces, so I have to add more cold, then the hot flow reduces, so I add more hot. It takes several minutes for the flow to stabilise. My wife on the other hand likes a generous shower and doesn't have the same problem.

This is expected. The rubber gasket inside the tap expands when exposed to heat and blocks the flow off. Since you open the gasket, say only 2mm, and the gasket expands, say 1mm, you have 50% blocking of the flow. Wife open the gasket 10mm, expansion eats 1mm, only 10%, no noticeable difference in overall water temperature.

Back to the wet drip: It could be possible that the very low water pressure (water head) makes the dripper highly susceptible to small temperature changes that happen as the water starts flowing? I'm also interested in setting up a very accurate drip for cold-brewing coffee over many hours. The setups one sees in that field are also low-pressure, but use a very accurate mechanical nozzle, certainly not from the irrigation suppliers. Perhaps you could look at one of those?

Hi freddo,
I've done a fair bit of hydraulic testing, usually on much larger systems, and its common for it to take hours for the system to get to equilibrium. With such a slow flow rate and a large tank it could take a very long time, and every time you adjust you need to wait all over again.
For this I'd suggest a peristaltic pump. Pretty easy to set up yourself if you can find a good variable speed motor. Handles dirty water ok. But... even then 5% accuracy will be a challenge. A cheap one off the shelf would be your best bet.

I have a solution - but I only do consultancies :-)

Do you have a consistent head of pressure? ie the volume above the orifice is constant a drop in volume will reduce flow due to the reduction of mass?

on a thought tho.