Today I've added my coins of Belgian Congo, so now I can give you an in depth answer to your question.
First of all, there is no H mintmark for the coins struck in Heaton (I've already made a change request to remove those H's).
But there are many differences between the coins struck in Brussels and Birmingham (Heaton Mint). Some are more difficult to see, like the 2 x 3 stars which are a bit bigger on the Birmingham coins, or the date, which is a bit smaller and thinner:

Or the difference in distance between the point of the star and the first line. Left picture is from Birmingham:
But the best ways for determination are those three (left and upper pictures are from Birmingham):
- The shape of the C:
- Amount of balls around inner hole:
- Amount of balls in the star points:
As you can, very easy to see whether it's a coin struck in Brussels or in Birmingham.