This paper investigates the relative frequencies of the two major syntactic markers of future time expression (FTE), be going to and will in the Diachronic Electronic Corpus of Tyneside English (DECTE). In particular, the rise in the frequency of be going to will be examined in the light of current theories of grammaticalisation. The various grammatical constraints that have been identified in the literature as determining the distribution of will versus be going to will be investigated. It will be shown that a number of interesting changes have occurred within the fifty-year period covered by the data-set. In specific areas of grammar, contrasts have been maintained (e.g. first person versus the other persons in the favouring of will), strengthened (e.g. subordinate clauses versus main clauses in the favouring of going to), weakened (e.g. the dominance of will in contexts of distal future reference) or even introduced (e.g. the apodoses of if-clauses emerging as a syntactic niche for the favouring of will).