Thank you for your help guys. The problem is solved. Perhaps if I had provided more information I may have gotten there sooner.
Here is the story. My dashcam is a Blackvue DR650. The viewing software for the PC is good for viewing and has an option to import the clips (MP4 format) but only individually. I tried several video editors, including WMM, and I was unable to merge multiple clips into a single seamless video. Also, I gave GoPro a shot but it just froze on my computer.
Eventually I came across a dashcam viewer called Registrator Viewer. It's a freebie download and has most of the functions of the OEM viewer as well as the addition being able to select multiple clips then save them as a larger, seamless clip. This was my goal. Now I can go back to my default video editor and, in conjunction with Resitrator Viewer, carry on with my project.
Give Freemake video convertor,a go when installing use custom, and untick a couple of programs it thinks you need if you just install it. I've used it over the years and its pretty good, it has a join feature and it's free...
quicktime pro can join HD MP4 video without rendering (which makes it extremely fast, and no video quality loss)
Just select the "passthrough" method which tells it not to re-render all the clips which loses quality (it virtually joins the files into one file instead of re-rendering, so its as fast as your hard drive can copy a file that size) . Its been years since I used it so I wont be any help with instructions on where to click but its how I used to join HD video together as it was so fast
Doesnt work with all files (the "passthrough" selection is greyed out if its not possible which means re-rendering is the only option) but it does work with HD mp4 files which is what I used to play with