Ok so after reading this book by Andrea Stolpe, I came across a process called object/destination writing.
It took me a while to understand exactly what she was talking about but once I understood it, it's been immensely helpful. It's not exactly helped with lyric writing as such, but it has got me writing and thinking in a more descriptive manner.
I've only been doing it for a few days so far but already I can see an improvement.
So obviously the next step will be for me to extract and adapt any possible lyrics from my writing but I think that's still a while away at the moment. I'm just happy to be able to write at the moment, something which is completely alien to me!
Just thought I'd mention it as it may help others who are having difficulty writing.
Here's a little bit more about it.