I don't think they're any more heterogeneous than the other parties. You could equally say that Labour has a strong Old Left nationalise-industry wing and a New Labour / Third Way wing, or that the Conservatives have a libertarian wing and a social-conservative wing. The most obvious split in the Lib Dems just happens to be on this issue (somewhat because they were two actually different parties until quite recently for a major party - it's pretty much the difference between the Liberals and the Social Democrats.)