Years ago we used to cook up linseed and barley for our arab to keep her weight on as she got older.
I feed instant linseed to my two. Chancer is only a baby - rising 3 and has it for his show coat. Cairo is our old clydie who is 21. Since he has had linseed his movement has vastly improved - no paddling in canter, spins on the spot and is sounder than he has been for the past two years - now completly sound on one rein and 1/20 on the other.
I have stopped using cortaflex and at the moment have noticed no change.
I get my instant linseed in a large sack from Simple Systems long with the rest of my feed stuff.
Linseed and barkey for older horses definitely. Linseed also keeps them regular as well as putting on condition and keeping the coat shiny. I still buy whole linseed and soak and boil, same with the barley.
i used linseed and fenugreek over winter on my 2 tbs and my mates elderly mare, they were all fed 100grams a day (2 normal vitamin supplement scoops) and held weight well, although the fact they were all on fibrebeet may have had something to do with it - my friends elderly anglo arab mare is out 24/7 and the addition of fibrebeet to her feeds made a huge difference in her weight... its the fattest she has been for years....