Thought I'd post how I solved this for any others who need help. If anyone has an easier way, feel free to post!
Open up Mapsource on your computer. Look at the dropdown menu on the extreme left of your Mapsource window. This shows the versions of maps available to you.
Now update your maps on Garmin Express and there should be an option to save to your computer or to your unit. Save to your computer.
Once you have done this (it will take a while, I left mine overnight) go back to Mapsource and look at the dropdown menu again. You should be able to see "City Navigator Europe NT 2015.4
Bingo!
Now plug in your Zumo to your computer. Go back to Mapsource and choose the "Send to Device" option. You should have an option to send maps to the memory card by using the drop down menu which appears once you click on "send to device".
First however, you must select the map tool from the menu at the top of the screen. This will allow you to click on the tiles / maps you actually want to send to the device. Easiest way is to reduce the map scale to something like 500 km. The tiles you select will be outlined in yellow and the centre will turn pink.
Once you have selected your maps, go back to the send to device option, make sure you have selected the option to send to the memory card from the drop down menu, click "Send" and job done or should be.