I think I may have gotten around it, by creating another function called U[x]:

U[x]:=subs(x=x(tau),z=diff(x(tau),tau),diff(subs(diff(x(tau),tau)=z,x(tau)=x,U),x))

Seems kind of stupid that I have to jump through all of those hoops to do something so straightforward. Surely there's an easier way?

That works great, thanks.

One further question; if I then wanted to return the value of r(tau) where tau equals 1, for instance, how would I go about doing that?