Many thanks SUroot.
I'm due for an upgrade but nothing seems to be dramatically better to justify an upgrade & I love the Desire. It's been driving me nuts though because although I have everything on it I want, with the updated maps it kept giving me the low space warning with only around 10mb free.
Now even with the latest maps I've 25mb free. Some things had to be moved back to the phone, like Swiftkey, but even Facebook has moved to the sd card without issue.
This is a great, simple solution without going down the slightly scary route of rooting; I need the phone for work so can't risk doing something that I might mess up.