The best products you can buy for your fuel problems are made by these companies:
Castle - Highway 95
B3C Fuel Solutions - Ethanol Shield or Mechanic in a Bottle
Gasoline Medic - Gasoline Medic or Carburetor Rescue
I have seen these products tested side by side with Stabil (all different variations), Seafoam, Startron, K100, and a couple others that I don't remember. I've used all of the brands I have mentioned, as well as sold all of the brands. The top 3 I listed are superior.
I'm not sure if the problem you have is the same as what we encounter here in the states, gasoline is regulated at 10% ethanol content. That along with age and moisture are the biggest problems. I feel more comfortable buying 87 octane and treating it than I do 89, 91, or 93 where available. They all have that same 10% ethanol, but the 87 is most likely sold through more commonly. Some of the higher octane choices, depending on your gas stations, may actually be older gas sitting in their tanks if it is not sold off as fast as the cheap stuff. Buying old "premium" gas isn't as good as "fresh" regular IMO. There's no way to know in either case, it's all a gamble at the pump today.