Some sound advice Biggus,
However, there is one small error in your text which is worth pointing out.
You said that:
Without going into the full details it all boils down to the fact that if you leave at 55 you get a single gratuity, there and then, if you leave before 55 you get one gratuity when you leave, and another at 65.
Actually, it should read:
Without going into the full details it all boils down to the fact that if you leave at 55 you get a single gratuity, there and then, if you leave before 55 you get an Early Departure Payment when you leave, and a pension gratuity at 65.
At the moment, both the Early Departure Payment and gratuity are tax free. However, only the pension gratuity is perhaps protected from tax because it's viewed as a tax-free lump sum of ones pension. It would take a major change of tax law to tax your gratuity, but it would be quite easy for the treasury to start taxing Early Departure Payments! So be warned!