I've currently got a single speed set up with a 32 : 16 gearing which is fine for off road but I'm now planning on using it as a city bike to get around town on and so need a more suitable ratio.

Trouble is I have a Middleburn RS 7 Uno crankset which has a combined spider/chainring and they only do a 32 tooth option. I've seen 14 tooth rear cogs, will that make a noticeable difference so my feet don't spin so much on the road or am I better off buying a spider for the cranks to mount a larger chainring?

Flatish off road yesterday with knobblies and gravel and I was running around 66 inches [gear inches that is] most of the time. So  if I was looking at a single speed road bike it would be with a gearing of 70+ inches.

44:18 would give 64

42:16 would give 72

So I would try Billys 42:16 to start.