I often travel to JER with friends who are not even blue, never mind silver or gold. On through-tickets from NCL with the LGW-JER-LGW sectors in CE, even then they are refused use of T5 lounges on the return trip. I would be surprised therefore if someone who is blue and who has bought a separate MAN-LGW-MAN ticket that is not UK business is allowed MAN lounge access. Seems odd to me.