Ok! So managed to have some success with this in the end - quite pleased!
Am now able to print out Form Instances directly from a mobile device to a Wi-Fi printer using the little print button when viewing a 'Sent Form':
It's definitely not perfect as it requires that the XSLT file is updated for each Form Template that you want to print out but we don't often create new Form Templates so not a huge issue for us at the moment.
I can share the code if anyone is interested once I've tidied it up a bit! (Ok, I don't think any amount of tidying is going to improve it - it´s over on the dev forum if anyone is interested; Printing a Form from ODK Collect using XSLT - #6 by chewDK)