The trick is not to use mp4 as the format for the export, but "mpeg transport stream files (.m2t)". You can find it by choosing Share > Create Video File > Custom and clicking on the drop-down menu beside "Save as type". Then you click the Options button and go to the Compression tab on the dialogue box which appears. Make sure you have correct TV system selected in the top box (PAL or NTSC). Then down below that, in the box beside 'Video format' change the default format from mpeg-2 to h.264 (which is of course AVCHD mpeg-4). Then tick the AVCHD box which appears below that. Then back on the General tab, you will find 50 fps as being one of the options under Frame Rate. You can also change the frame size below that to whatever you want.
Then click OK to close that dialogue box, give your new file a name and click Save to close the original dialogue box and begin the conversion.
If you intend to do this regularly, you can make your own Custom template using Settings > Make Movie Template Manager.
I guess you could also try Share > Create Video File > Same As First Video Clip", but I don't think that will work.
Note that VS is currently limited to a maximum bitrate of 18 Mbps for AVCHD, rather than the new international standard of 28 Mbps. The end quality is still very good quality high def video, but not quite as good as it might be using the new standard rate.
