Firstly I have a geared bike so no 'get gears' responses please…

Wondering if anyone runs 2 cogs on their single speed – one for road work and one for offroad and manually adjusts the chain tension (slot dropout) as and when you wish to swap ?

32.18 is a good ratio for my local loop, however it's a good 5-6 miles on the road to get there and it's really boring spinning away.

Anyone?

I've seen a few people do similar

You have a valid point. You are describing a dinglespeed. The conventional way to do it is to have 2 chainrings and 2 cogs. 32 and 36 up front, 18 and, say, 14 at the back. Then you have 32:18 and 36:14, similar chain lengths and good chainline. Perfect.

I'd never heard of that before – off to do some googling

voilà

e.g. a WI DOS Eno freewheel?

http://www.whiteind.com/singlespeedgearing/freewheels.html

Or a flip-flop hub?