Thanks all for the replies and info, especially to LizzieL for the new leads.
I know that with records of this age there is often an element of 'best guess' as the detail recorded is so scant. It's also further complicated in that there appear to be a number of Gray families living in Great Marlow around that time and into the 1800's and there does appear to be two or three generations of Robert at various stages and in at least one occurrence, both married someone of the same name.
Going through things though, unless I've missed something, the most likely scenario based on the records found so far is that:
Robert Gray was baptised 04/01/1757 to parents Robert and Elizabeth
Ann Allum was born c1752
Robert married Ann Allum 23/02/1778 Robert aged 21, Ann aged 26
Ann Allum was buried 10/11/1816
Robert was buried 28/05/1837
All the dates do seem to match up. The thing I'm less certain about is Robert only being 21 when he married when Ann was 26. Whilst certainly not impossible, it just 'feels' a little too much of an age gap for people so young.
It's entirely possible that Robert wasn't born in Great Marlow, it's just that 3-4 generations after him were all from there. Indeed I'm really struggling to trace this line any further back than this.