Hello sparerib and Paul, what you need to do is copy the gpx file of your trip from the Lowrance SD card onto your computer. I use Garmin Homeport (which crashes on Macs) Garmin Basecamp, Memorymap and Anquet Outdoor Map Navigator on my computer which all read gpx files but you need to buy or subscribe to maps for these to work but you can either get your track to appear on marine charts or OS maps, whatever you have paid for:
Anquet OMN OS 1:50,000
Memory Map Bitmap Admiralty chart BA2094 1:100,000
Garmin Homeport Vector Bluechart
There is a free work around. Convert your gpx file to a kmz file using gpxvisualiser web tool
then import the kmz file into either Google Maps or Google earth
in google maps
>log into your google account
>import (drag the kmz file into the box that opens)
Google maps base map
Google maps satellite
Of course I could not possibly recommend you search BitTorrent sites for free maps as that would expose your computer to all sorts of nasties.
You don't need to have a Lowrance or other GPS unit. There are plenty of Android and iOS apps that will use your phone GPS to create a gpx track. Of course don't expect your phone battery to last long with the GPS running!
Have fun, Douglas