Thanks JW411
That makes lots of sense.
Like I say, I never fly FL due to being VFR around Farnborough (not above 2,500ft mostly) and the weather.
I was just some what confused by the RT I heard. Although it was after a 12 hour flight so couldn't definately say there were clearences of 17,000ft there were certainly altitudes of 5,000ft being banded around. Your post explains why!