  The null limits of the Lemaitre-Tolman and Szekeres spacetimes are known to
be the Vaidya and news-free Robinson-Trautman metrics. We generalise this
result to the case of non-zero $\Lambda$, and then ask whether the reverse
process is possible -- is there a systematic procedure to retrieve the
timelike-dust metric from the null-dust case? We present such an algorithm for
re-constructing both the metric and matter tensor components of the
timelike-dust manifold. This undertaking has elucidated the null limit process,
highlighted which quantities approach unity or zero, and necessitated a careful
discussion of how the functional dependencies are managed by the
transformations and substitutions used.
