For some reason Lightroom has started to create a 2017 subfolder inside of my 2017 folder- meaning that everytime I now add photos it's adding them to a subfolder called '2017' within the 2017 folder. Can't figure out why on earth it is doing this and how to fix it... any help appreciated! (See image below).
It's happening because you have the wrong folder set as the destination folder, You have got the 2017 folder selected and you have chosen to organise by date using a yyyy/yyyy-mm-dd system, and that directs Lightroom to create a dated folder structure of, in your example, first a 2017 year folder and then 2017-03-18 and 2017-04-08 sub-folders....all under your selected 2017 folder. Hence you'll get a new 2017 folder under the existing 2017 folder.
The solution is simple....change the target folder from "2017" to "Pictures".
Ah-ha! Perfect- Thank you!!