I use Dettol. The combination of the isopropyl and pine oil is apparently what does the job (I think someone said that Lidl do an own-brand version with the isopropyl and pine oil, which is cheaper).
You soak them overnight immersed in it and brush with a toothbrush. Don't rinse with water though until it's all gone.
Some people don't like the Dettol smell and it isn't a quick method. The advantage is it's cheap (I think Wilkinson's gives the cheapest deal and you can filter it through kitchen paper and reuse it for paint stripping again), it's relatively safe on the skin and can also be used on plastics with a little caution.
"You're a big man, but you're in bad shape. With me, it's a full time job." – Lt. Bromhead to Prince Dabulamanzi before the Battle of Rorke's Drift.
Not sure if you can get it where you are, but brown dettol definitely works! I've cleaned hundreds of minis, both metal and plastic with it and the results are fantastic even on the terrible 20 year old enamel paint jobs I did as a nipper. A toothbrush will remove any stubborn bits left over in crevices, obviously. The only downsides are that it weakens glue on plastic minis, and the smell will tend to pervade a bit. Sealable containers come in handy. It works incredibly well and doesn't seem to damage either metal or plastic though.