Hi Graeme, in principle the simple answer is "yes".
However, in order for this to work your NAS box must have an address that's visible to the Internet, so in practice there are a few other things to consider.
Q1. is your NAS visible (on port 22) from the Internet (if you have a standard home ADSL router / account, this is likely to be "no")
Q2. does your NAS support file sharing via SSH (it sound like this is a "yes" from what you've said)
The "usual" solution here is to log in to your router, and tell it to forward all incoming traffic from the net to port 22 on the local address of your NAS box .. "how" you do this will vary greatly from router to router. Assuming the firewall on your router is allowing port 22 (or assuming you modify the router to allow port 22) then this should "just work".
Warning :: anything visible to the net is hackable - if you use SSH with a 'password', you may not want to do any of this. Normally under such circumstances you will use SSH with a 'certificate'. On your Linux box you can use "ssh-keygen" to generate certificates, by default these will be created in your home folder under .ssh/id_rsa and .ssh/id_rsa.pub. The .pub file will need to be installed on your NAS, and the id_rsa will need to be installed on your laptop or whatever other device it is you're using to access the NAS from the net .. then you need to turn SSH password authentication off on your NAS. This will be about as secure as you can make it short of not sharing it over the net ...
Note :: most ISP's will issue "dynamic" IP addresses, so each time your router connects to the Internet it will get a different address, the effect will be that the address of your NAS will change from time to time - which could make life difficult for you (!) I would recommend paying a little extra and requesting a "static" IP address, in this instance your router's IP (and consequently the IP of your NAS box) will remain the same. BT charge an additional £5 a month for a static IP on their BT infinity product, whereas Zen Internet include one in the price (last time I checked) ..