The leader of the opposition, (in the manner he had planned to convince them,on) P (realising that he had failed to convince the) Q (assembly,who had a reputation for speech) R (making,was very much disappointed) S The correct sequence should be

Since, the sentence starts talking about the leader of the opposition, the connecting statement will be R as it carries a relative pronoun ?who? which relates the sentences. S follows as it states the disappointment of the leader and the next statement talked about the reason of disappointment. P would be the concluding statement as it goes with the flow describing how the leader decided to convince the assembly and thus makes a coherent paragraph.

The correct formation would be, ?The leader of the opposition who had a reputation for speech making was very much disappointed on realising that he had failed to convince the assembly in the manner he had planned to convince them?.