The small parties have to reach the five-person threshold. This is very important point because it's connected with the point of strategic voting. I think the most important failure of the first past the post system is that it discriminates between votes depending on the preferences that are expressed in the vote.
In the first past the post system, the design was intended to reward strategic voting, that some citizens give their votes to parties that are not their preferred party. Without the strategic adaptation to the electoral system, plurality systems couldn't produce the desired manufactured majorities of a single party. This means that the cost of a sincere truthful vote, which is in accordance with your true preferences, depends on your political convictions. If you aren't very happy in situations and your preference is for one of the candidates of the two biggest parties, your cost of casting a truthful vote is equal to zero, because this is exactly what you should do. However, if you are a member of the Green Party, in most constituencies the cost of a truthful vote is high, because for a truthful vote you have to sacrifice your only chance to have any impact on the electoral result.
The point is that there is no system without strategic voting, and strategic voting is needed to make systems function in the way they're intended to function. To give the voters of small parties a chance for a bigger percentage, we have a level that is low enough, at 5%, and also high enough to prevent a total fragmentation of parliament.