The Formula S18 is a pretty cool bike with the disc brakes and double up front ... shame to dump because you can't get low enough gears .. I'd be inclined to run a mega-range 11/34 or 12/34 ( if you don't need 96 g/i on top) and a longer cage rear der .. you'll cover the extra wrap and can leave the front alone (if it really is the case you can't put a 39 up there for some reason).. I'm running a 60/39 on my Moulton and the Shimano front der handles it just fine.. my first order of business would be to mount a 39 up front and make sure it won't work .. that would give you a 29 g/i and maybe low enough.. if not, then on to mega-range rear cassette.. I wouldn't give in just yet..