I've searched for information on how to upgrade from 1.1 but I've not found any results so I'm wondering if it's worth me updating and if so how should I do it?
My server runs on Apache and Ubuntu 12.04.
Varnish 1.1 is quite old at this point. Ubuntu 12.04 ships with 3.0.2 by default. While version 4 has been released in the meantime. There have been quite a few changes to the VCL file syntax. While there aren't any official docs for upgrading from 1.1, there are some covering the changes in 3 and 4:
https://www.varnish-cache.org/docs/3.0/installation/upgrade.html
https://www.varnish-cache.org/docs/4.0/whats-new/upgrading.html
You'll most like have to make a number of changes to your configuration. It might be best to start clean.
If you don't know why you'd even upgrade in the first place, it's probably not worth upgrading.
I'm not sure why that would be the case? I not up to speed on the latest versions of Varnish and I thought someone else who is may be able to help as I wasn't able to find any easy to understand documentation on the changes since 1.1.
If you're on 1.1, you compiled it yourself and I assume you have a good reason for running that version, since it's from 2007. If you're using the Ubuntu packages, you're almost certainly not running Varnish 1.1 and I don't see why you'd upgrade from the packaged version.