The only slight difference I can perceive is that "spare time" carries meaning of time between things - i.e. you have "spare time" between work and going to bed, but "free time" refers to longer stretches such as weekends. That's just a little thought in my head though, they are for all intents and purposes interchangeable.

they are the same thing basically

More British but not exclusively because occasionally you hear Americans and Canadians use it.

In my spare time I usually watch old films or work on my car. In my free time I usually watch old films or work on my car. The same meaning. If you have some some spare time this afternoon, could you come over and give me a hand? If you have some some free time this afternoon,could you come over and give me a hand? Give me a hand= Help me to do something. Again, they have the same meaning. 'Spare' is more British

No difference really!