What Briggsy's trying to tell you, Rugbyluke, is that your bike has vertical rear dropouts, and so is unsuitable for conversion to a fixie.
Why? Because chain tension is crucial on a fixed gear bike. Getting that tension just right means you must be able either to move the rear wheel relative to the chainring - by means of horizontal dropouts - or the chainring relative to the rear wheel (by means of, say, an eccentric bottom bracket).
On a dérailleur-geared bike, the rear derailleur's springing keeps the chain wrapped around the rear sprockets and tight, so there's no need for fine adjustment of the rear wheel's position.
On a single-speed bike, you can use a similar derailleur, locked in position, or a chain tensioner, to maintain tension, without having to adjust the rear wheel's position.
On a fixed-gear bike, this won't work. I'll attempt to explain why, but it's kind of tricky, and you probably won't understand until you've ridden one for a bit, but here goes... Taking away the freewheel changes things fundamentally. When you're freewheeling on a bike that's
got a freewheel, the transmission's doing nothing. No torque is going anywhere through the transmission. In the same situation on a fixie, the transmission is still in action, only now the torque's going the other way - all the momentum of your weight and speed and the bike's are pumped back through that chain to your legs. You can resist this to slow the bike down
. But not if there's a chain tensioner - that will only work with the transmission going forwards. If you try to resist on a fixe with a tensioner, the tensioner would fold up - and probably break. Then the chain would come off, potentially jamming the transission.... and things become expensive very quickly.
All that torque coming back through the transmission at you is quite fearsome. You do NOT want anything going wrong with the transmission. If it does go wrong, it can be spectacular and very, very dangerous.
No, fixie chains need to be properly tensioned (and treated with great respect). For this, a suitable frame is needed. I'm afraid yours looks to be not suitable.
But fear not - you can find a really hip old steel frame with horizontal dropouts on
Ebay for a few quid and transfer your other parts - or just build a complete new bike. I did.
