pb84,
This is what you are looking for:
10 When two aircraft are approaching head-on, or approximately so, in the air and there is a danger of collision, each shall alter its course to the right.
Just to add to the confusion, the same turn-right rule applies on the ground if you are in a "flying machine", but you turn left to pass another "flying machine" on the ground, whereas in the air you turn right!
PS Not a stupid question. Having learnt to fly in the UK, I thought I knew the rules, but I had a big surprise reading the "on the ground" rules. Who makes up this stuff? Vehicles are supposed to turn left in a head-on situation, flying machines turn right. What happens when a vehicle meets a flying machine head-on?