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@hilarystone
It is possible to convert a standard threaded rear hub hub to take a fixed sprocket. I showed how to do it back in Cycling Plus 147. And I think there's a load more info Sheldon Brown's website...
But here goes with the text - it is quite a bit of work...
1 Converting a screw-on freewheel hub for fixed sprocket use
Screw your single sprocket onto the hub of your rear wheel. Loosen off and remove the locknuts on both side of the hub. Remove spacers from the right-hand side of the wheel so that when a single sprocket is screwed onto the threads it’s centre line is approximately 18mm from the outside edge of the locknut when it is refitted. Add the surplus spacers to the other side of the hub so that the over lock dimension of the axle is 120mm or to suit the width of your frame. Carefully tighten the sprocket side locknut against the cone.
2 Adjusting the hub bearings and redishing the wheel
Adjust the cone until the wheel spins freely and the axle can turn easily without any resistence. Tighten down the locknut. Check that the axle can be rotated freely. If not readjust the cone and locknut’s position until the adjustment is correct. The wheel will now need some redishing to ensure the rim is exactly centred between the hub locknuts. Initially slacken the spokes on the sprocket side by about two turns and tighten the spokes on the opposite side by an equal amount. If you don’t have a truing stand and dishing tool most shops can redish the wheel correctly for less than £10. (probably £15 or so now)
3 Checking wheel dish
With the dishing tool check that the rim is perefectly centered between the hub lock nuts. If the dishing tool’s arms hover above the rim the or the pointer/measuring sleeve do not reach the locknut, the rim needs to be moved. Carefully note the direction and initially turn all the spoke nipples on that side of the wheel by half a full turn. Slacken the spokes on the opposite side of the wheel by a half turn also. Recheck with the dishing tool and repeat making smaller adjustments until the rim is perfectly centered between the hub locknuts. Finally make any small adjustments to the spoke nipples to ensure the wheel is true.
7 Chainline 1
Correct and accurate chainline is essential on all fixed wheel bicycles. The front chainwheel must be exactly in line with the rear sprocket. To check the chainline sight along the chain from the rear sprocket. The chain should leave the front chain wheel centred exactly over the chainwheel's teeth. Alternatively run a long straight edge along the chainring; it should line perfectly up with the rear sprocket. It is possible to add spacers behind the sprocket (or bottom bracket fixed cup if an old style bracket is used) or to turn the sprocket around to help get the chainline correct.
8 Chainline 2
Mounting the chainring on the inside of the crank's spider can help and remove the necessity for a new shorter bottom bracket. If the bottom bracket is the old style type with separate cups, try reversing the axle as the left side of the axle is shorter than the right. When you are satisfied that the chainline is correct, remove the rear wheel and tighten the rear sprocket really tight with a good chain wrench and an extension tube to give a bit of extra leverage. Screw on an old style bottom bracket lockring and tighten this hard against the sprocket; it will help to prevent the sprocket unscrewing. Most track riders do not in fact use lockrings!
I would also use a medium strength Loctite thread compound if you are unsure...
Hilary