Pretty unsettling when that happens - the way it just sits in your head afterwards is almost as strange as the sighting itself.
First thing I'd say: write everything down now if you haven't already. Time of night, duration, colour, movement, sound. Memory degrades fast and you'll be grateful you did.
A few questions worth thinking about:
Was there any sound at all, or completely silent?, Did it move in a straight line or more erratic?, Any effect on animals nearby - dogs kicking off, birds going quiet?
I'm up in Lancashire and we get the occasional odd thing over the fields round here. Most of it turns out to be nothing remarkable but every now and then something genuinely doesn't fit any obvious explanation.
If you've got a decent camera setup it's worth pointing it out there over the next few nights. I use a Sionyx Aurora for low-light stuff - picked it up after a particularly odd night about three years back. Not cheap but proper useful for this sort of thing.
What direction were you facing roughly? And was this a one-off or have you noticed anything unusual out there before? Sometimes a single sighting is the last piece of a pattern you hadn't noticed yet.
Would be interested to hear more detail from others who've had similar - especially anything over agricultural land. There's a definite cluster of reports around farmland that I don't think gets enough attention on here.