Before I exchanged the 315 for a 530/5, I asked Swift for a drawing of the roof construction for our 2002 Eccles Onyx. They very kindly sent this which showed that there is large wooden block in the roof to support it. I removed the 315 and fitted a blanking plate, then drilled a 44mm hole through this and the roof. The plate held the hole saw in line. You must ensure that the hole is vertical. The only snag I had is that the Onyx has double the depth of roof due to the TV cabinet. However, the clamp nut does JUST grab the pole. I also needed to modify the panel with the 240v/12v/aerial sockets to allow access.
Works very well and no need to set up an external aerial.
Mike