Luckily I dropped Openelec over the weekend again and moved back to Win 7. Too many small issues with Openelec for me.
Thanks for the advice. Will search around the web for a suitable app.
IIRC Windows 7 actually has this function built into it in the sound setting.
EDIT: Just checked, and it's there - right click the volume icon in the system tray, go to playback devices, click on properties>enhancements>tick loudness equalisation and then test it out.
If it's not quite working then there are third party programs that can do this - some actually manipulate the volume control automatically - you can watch it move up and down on its own. Entertainment for 2 minutes or so...