I have a 56cm Coppi frame sitting in my shed and I am thinking of building a single speed.
I was wondering about having a flip flop rear wheel with two differing cogs for different uses hills/flats.
But I was also thinking of running a double chainset up front.

Any advice on which cogs to use and whether a double chainset would be worth using and what numbers of teeth on each ring ?

Thanks for any advice.

if you've got a double front ring you'll need a chain tensioner at the back – what's the advantage over running 1×9?

Sounds like you don't want a singlespeed at all 😉

If the difference between the two chainrings is the same as the difference between the two sprockets, you shouldn't need to adjust the rear brake much (or at all) when swapping between beg-little and little-big.

So, running 42:16 and 38:20 for example.

holly, you seemed to have misunderstood the whole singlespeed gig.

If you want to have two different gears, you could use a Surly Dingle Cog, along with a double chainset. http://surlybikes.com/parts/dingle_cog/

It means you don't have to turn the wheel around. Plus, as miketually says, if you have same tooth difference on the sprocket and chainrings, then the wheel position / chain length doesn't change.

What you want is a Sturmey Archer S2 duomatic (two-speed kickback hub)!

http://www.sturmey-archer.com/products/hubs/cid/7/id/57

Just man up and run a decent ratio.
You'll soon develop a pair of engine turned legs and a larger than healthy appetite for retro jerseys.

Holly. Just run it singlespeed (or preferably fixed if it's for the road).

Pick the same gear that the mainstream use. Check Langster etc.

If it's wrong, fettle it later.

Defo do not have two chainrings or any of that silliness.

You will not regret going fixed.

As said above, 2 ratios will be very handy eg. one for on road and one for off (or if it's just plain hilly), if that is your plan.

If you don't want to go 'fixed' at any point I have (for sale) a flip/flop Surly 1×1 free/free wheel where you can have a different ratio each side. White Bros also do a duo freewheel but it is on the pricey side.

My Surly wheel for sale HERE

If you have vertical dropouts, I'd be wary of running it fixed – I've pulled the axle forward in my track bike before* – I'd have been done for if it had vertical dropouts.

SS with a freewheel would be better. To avoid issues with trying to run track wheels in a road frame, use a normal road wheel and a slide on SS cog. Position the cog to get the best chainline using old cassette spacers.

You will likely need a chain tensioner to take up the slack in the chain, unless you can find the "magic" ratio.

Chainset – If you want a cheap dedicated single ring set-up, try the stronglight ST55 chainset (I use one on my road fixed).

*cue rear wheel jamming against chainstay and locking up and me having to point the bike straight down the banking at Manchester – managed to ride it down to the concrete without falling off or taking anyone else out. New shorts time..!

Ah never thought of of the drop outs.
I have vertical drop outs like a normal racing bike.

Will this affect me running a single speed setup ?

yes, your chain will either be too slack or too tight unless you;re extremely lucky and even then, the chain will 'stretch' and so it will eventually be too slack. You need a tensioner witrh vertical dropouts and if you're superhuman like our maninthenorth, horizontal dropouts and a chaintug.

I had this conversation with the guy in Bristol that I would need a chain tensioner.
But I was not sure of exactly what I need.
I may as you guys say just use a single on the front and a single on the rear.
But I am un clear what is the norm for gear ratio's

But I am un clear what is the norm for gear ratio's

There's no 'norm', go with what suits you best. How about starting with a samllish front say; a 44 or 48, and something like between 16T or 13T on the back. Give you a goodish speed on the flat on not too mad for hills (unless you're going to tackle properly steep stuff). old dérailleurs make excellent chain tensioners, unsurprisingly.

normal on the road would be 42:16 or something around that. often you will need to experiment to see what suits you.

48-13? I'm starting at 48-20!

48×18 on a 700c wheel, thought it was too spinny at first but it's perfect for commuting, can still winch up some bonkers steep stuff too.