Thanks all. It's funny, I put this track on the UK songwriters forum, had plenty of responses but almost nothing about the vocals being too quiet. I completely respect you guys' opinions, and I specifically asked for mix feedback, rather than song feedback. It's always easier to "hear" the words if you are reading the lyrics, which were published on the other forum.
The other possibility is that like many others I suspect, I am not hugely confident of my own voice. Therefore, I might think the song sounds fine, precisely because my voice is lower down in the mix. Makes for a more comfortable listen, but that's only to my ears! So, that's why I need detached opinions...