3.7kg of concentrates [altogether] is ok, but ONE BISCUIT of hay per day?!
My first move with ANY horse I get is to put it on bulk hay. My two are presently on 24/7 access to a BIG round bale of high quality oat hay.
Horses HAVE TO eat 2% of their ideal body weight or more per day in roughage. For my 550kg gelding, that's 11kg of roughage EVERY SINGLE DAY. Between pasture and hay. You can ONLY get away with only one biscuit of hay a day if you have a lot of pasture feed, or an exceptionally easy keeper [and even then, you should give them MORE hay, that's got LESS nutritional value].
For the older horse, especially, more roughage is very important, if they have enough teeth in their heads to eat it. If they don't, beet pulp is a good solution for forage replacer [speedibeet is a very good brand, it only takes 10 minutes to soak up, 5 if you soak it in hot water]. You can get a thing called fibrebeet from the speedibeet people which has lucerne added into it as well, and that's great for horses that can't eat hay for whatever reason.
By the terminology you're using I suspect you are a fellow Aussie, and therefore in summer time, meaning your pasture feed is unlikely to be sufficient to keep your horse's roughage intake up.
Keep it simple for the older gut. ONE concentrate is ideal. Two, tops, if you can't get enough into your horse of just the one. Gumnuts are NOT the best senior feed out, either... they are ok but not the best by a long shot. I had great success with Nutririce Veteran, with my currently 17yo gelding, but the price is prohibitive. Copra meal was also good, but supply is inconsistent. I currently feed more of a young horse ration, with a good 2.5kg or so of grower and weaner pellets and then another 1kg at least of flaked lupins [lupins are a legume, and legumes are AWESOME for horses]. I feed very little chaff, because my horses have full access to as much high quality roughage as they want.
First step, ALWAYS, is MORE HAY, until the horse will not eat any more... THEN you up the grain, or look into internal issues.
edit for poppy's benefit; a biscuit is what Aussies call a flake of hay, it's really not much!