Engine won't start

I've had this S70 for about 5 years and its never let me down...until now.
It's a 2.5 litre diesel turbo/ auto box, with about 170k under its belt.
Ordinarily it starts first time.
Last month, after a normal run, I came back to the car about an hour or two
later. I start the engine, it turns....but nothing. No detonation at any
time. I'm not sure if it's my imagination, but the engine seemed to be turning
over faster than usual..but I wouldn't bet my life on it! I repeat this several
times over the course of a few minutes. Finally, without any warning, it then
bursts into life...as if nothing untoward had happened.
The same thing happened today! Strangely enough, it was in the same place
as the previous time: way out in the country and far from any help.
I've been "lucky" so far - the car has always started, in the end. But it's a
stressful few minutes while it tries to start. Maybe next time it won't....
Any suggestions would be much appreciated!