The actual roller bearings have seals built into them so you can't do anything but replace them, Ian.
The cables should be stainless steel in nylon lined outers so are effectively non-serviceable. If they are nylon lined, don't use oil to lubricate them as it can react with the nylon liner and make the situation worse. Try something like a teflon based lubricant as sold for use on cycles. Again, if you're in any doubt about them, replace them. They're not expensive. I replaced a set on my Lune Whammel trailer (Knott braking system as opposed to Alko) but you shouldn't be looking at much more than £20 or so for a genuine stainless steel cable. If you do replace them take along your old ones to compare lengths - there's an assortment of different overall lengths for the inners and lengths of outer cabe to choose from.
Inside the drum the only serviceable items are the brake expanders and shoes. The expanders/adjusters are simple to work on and well worth cleaning thoroughly and greasing. The adjusters for the shoes are an appalling design and hark back to the 50s where a screwdriver has to be wielded through a tiny hole in the back plate to move an invisible wheel in minuscule steps
In comparison the Knott brakes are simplicity itself to adjust with a spanner.
Set the brakes up first using the adjusters though before altering the linkage to the towbar. You'll know if the linkage needs altering after that if when you brake a little more heavily than usual you get a pronounced thump from the hitch as it reaches the limit of its travel before the brakes are fully applied. The position of the handbrake lever is also an indication. The brakes should be fully on long before it reaches the vertical position.
Steve