There can be slight differences in axles made by different suppliers. That said there are two axle lengths for Spitfires, the short for the body style with the small tail lights and the long which was phased in for the 71 and later body style. The yoke does need to be fixed in place because the axle is a suspension locating arm on the swing arm suspension. I assume your suspension is still leaf spring and one locating arm on each side from the body to the outer assembly? If it has been modified to multiple links each side then you have a engineering project that you will have to sort out.
You will probably need to measure the length of your spare and the broken axle from flange to the cup on the yoke to see how close they are. They should be uniform but a small variance should probably be OK on a primitive suspension like this. If you are assembling and the axle seems short, either it is short or the bushings the connect the suspension upright to the bearing hub are not moving freely. The upright should pivot freely at the bushings so the axle can move inward and outward during suspension travel. Only the axle length is fixed.