As far as I can see, Titan do not include the Web Mk3 in their lists, but I believe the spec for the spring (taken from the Cobham book) is: Length 210mm, wire 3.25 diameter, id 15mm, 29 coils.
The Titan Ts001 is pretty close but would need cutting down a bit, I think.
Or I think a HW77 spring would fit.
Note, however, that with any ' after market' spring there is no guarantee that the spring guide would fit; although I suspect there would be no problem with either of the springs above.
Cheers, Phil