I have similar issues with doing work for landlords and companies with multiple sites. I usually just enter the actual site/property address as a comment line in the invoice. You can just select a previous invoice and then copy it to save retyping every time, depending how many sites and invoices/estimates are generated.
It would be handy to actually enter this into an address list in Quickfile though, but sometimes I have multiple sites on a single invoice so the comment line method works best for me anyway usually.
@QFMathew these topics seem to be existing feature requests for basically the same thing, pretty sure there is another one but my memory isn’t what it used to be;
http://community.quickfile.co.uk/t/delivery-address-site-address-on-sales-invoices/9403
http://community.quickfile.co.uk/t/site-address-needed-on-invoices/2609