I'm sure that you're correct in saying that this is often what we see (though perhaps with a capital "L" on Ltd. and Purchaser spelt with an "e"?) But is is right? It looks like word-for-word French to me....XYZ ltd, having its registered office at wherever, with registered number 1, hereinafter "the Purchasor"
An elegant solution.... I frequently take it out altogether and go for something like:
XYZ ltd, having its registered office at wherever, with registered number 1, (the "Purchasor")