Hi Kevin,
There is a way you can display UTR number on an invoice, albeit it's a bit of a work around.
What you'll need to do is head to Settings > Invoice themes and click the edit button for one of your custom invoice themes.
Click the labels link, then head to contacts subheading and change one of the fields there such as fax to read UTR, as illustrated in this screeshot: http://prntscr.com/5b0g2u.
Then you need to head to a customers edit contact page and in the other informations page, enter the UTR number for the fax field. as illustrated in this screenshot: http://prntscr.com/5b0hde.
Then when you create an invoice for this customer, using the amended invoice theme it will show on the invoice the UTR number as such: http://prntscr.com/5b0i2b.
Hopefully you'll find this an easy way to show UTR on an invoice as it doesn't require any complicated CSS editing, although you do sacrifice the ability to include the fax number on the invoice.
If you need any further assistance, please don't hesitate to ask.
All the best,
Shaz