Question…….

Front mech is in the big ring (2 presses to get it there from the granny), 1 press on the down shifter and the mech jumps down to the granny with an almighty thump.

Whagwann?

Road bike? Mnt Bike? Mech type? Shifter? A fair bit of info missing for people to be able to help.
I’ll be happy to put on my thinking cap of 15 years(!) in the trade 😉

Mountain bike, SRAM X7 top pull mech, shimano slx shifter. I’d been informed that it doesn’t matter about mixing Sram and shimano front mech & shifter.

Cheers normski!

Try having a look at this helped me
Front Derailleur Adjustment

1. In the granny ring to largest sprocket on the cassette check the chain just clears (3mm) the inside plate on the mech cage – adjust with low limit screw
2. In the middle ring to largest sprocket, check the same clearence again. Adjust with the barrel adjuster on the shifter
3. In the largest chainring to smallest sprocket on the cassette check the outside of the cage just clears the chain. Adjust with H limit screw.

Check that your cable is clamped in the correct part of the mech. If you route it around the wrong side of the cable clamp bolt it can alter the leverage ratio/throw of the mech. Or you might have your outer limit screw set too close so that the mech is fighting against it when you shift up to the outer ring. If this is the case the mech will have loads of pre-load on it that will make it pretty fierce on the downstroke when you shift down.
Pretty hard to diagnose over the web but these two things are fairly common set-up issues.
Good luck

Hmmm, I’ll start with checking the H screw and then the cable. Cheers for your input everyone!

Wrong cable routing. What a donut.

Knew it! 😉