Well, since no-one had any suggestions, I had to find out how to do it myself
It is possible to syncronize say a laptop to a desktop system, but you have to select the method carefully. This is how I sync a laptop I use on travel with a desktop system at home complete with external 500 GB USB2 drive.
1. Import your pictures on the laptop, sort, crop, tag etc while the memory is fresh.
2. When you get back to your home system, select the folders with the new shots and export them as a catalog TO A TEMPORARY DRIVE/Location. As you can not export a catalog to the same directory twice it is better to use a scratch disk.
3. On the main system, Import the newly created directory from the temp drive and COPY TO A NEW LOCATION - this should be your main image drive/location. All edits and tags, as well as the original directory names will be imported to the new location.
Done.
As my travel (actually my wife's) laptop is a MacBook and the main system a Windows system, my image harddrive contains a FAT32 partition for temporary use and a NTFS partition for the main image directory. The reason is, that OSX can not write to NTFS drives. (but it can read)
Regards
Michael