I have a fully battened mainsail, the leach flutter between the top two battens is terrible when the wind picks up.
I have two jammers for leach line but no leach line .
the sail is not a good brand is this a sign of a badly cut sail or is there a way the battens can be adjusted to fix this ?.

Old or new sail? Cut for your boat or "acquired".

If old sail either figure out a new leech line yourself or take it to a sailmaker for a service - I just did that with mine for new battens and got a few other bits sorted too.

The problem is self-perpetuating - all the maintenance guides from sailmakers tell you to prevent leech flutter with the leech lines to prevent wear.

info.sailexchange.com.au/hubfs/downloads/Sailcare-guide-v1.pdf

From North:

Flutter. Leech or foot flutter is flogging's little sister. When just the edge of a sail flutters, there is significantly less sail area moving, but the number of cycles is significantly greater. Also, because a fluttering leech or foot edge does not shake the boat, it is easier to ignore. But when you ignore it, it doesn't go away. Rather, the fluttering action steadily breaks down the fibers. Throw a little UV degradation into the mix and eventually the leech or foot tape will rip off the sail.This is what happens after many hours of leech fluttering. The fabric right at the edge of the leech tape has been subjected to thousands of cycles of bending back and forth, compressing the fibers at exactly the same spot each time. Throw a little UV degradation into the mix and the tape will zip right off the sail.Leech flutter is caused by unstable airflow over the trailing edge of the sail. If the air is flowing off the trailing edge fast enough, the force of the airflow will deflect the loose edge. That deflection will cause a vacuum that the fabric will snap back into and then the cycle repeats itself. The fluttering edge makes a noise like a playing card against the spokes of a bicycle. Treat this noise as a signal to get up and tighten the leech line.

Racing sailors are taught to ease the leech cord as much as possible to minimize the amount that the leech hooks to windward. The same idea is valid for cruising sailors, but we might as well err on the too-tight side in anything except the lightest wind. Better to have a slight hook in the leech of the sail than a ripped leech.Tips to control leech flutter:Position jib leads or the main traveler car so the sheet angle lines up with the leech.Tension leech lines properly.Excerpted from The Complete Guide to Sail Care and Repair, by Dan Neri

Look at mast bend up top. I would guess tension the back stay to add mast bend to flatten the sail and take up some of slack. Also if your budget covers it take the sail to a sail maker who may be able to trim that part of the sail. It may be stretched or if JonE as says it may not have been cut for your exact boat. If you can get someone to video your boat under sail (recon) it will be easy to tell versus guessing.

Thank you guys for the input, I have a hydraulic back stay and I am now wondering if I have put on to much pressure trying to flatten the main in strong winds , not that I thought it would do much as it is mast head rig , but it may have been enough to let tension off the sail letting it flutter at the leach . sail wasn't cut to take the mast bending.makes sense as the sail looked pretty good until then, you never stop learning.

The problem is completely the fact that there is no leech line. You'll need to have one installed. It may have broken somewhere and the tail has been pulled out of the sail. I'm a Sailmaker btw.