Steve
You need Wugfresh Developments Nexus 7 boot/root kit, a reasonable internet connection and a windows PC to connect the Nexus to.
What you are doing in essence is putting the Nexus into "developer mode", authorising it to accept O/S uploads via USB and then installing the earlier operating system.
Viola, your Nexus is restored to full functionality. The blasted thing will start offering to "upgrade' the O/S from about 5 minutes after you reboot - but you, I and tens of thousands of others know the truth about lollipop on a Nexus 7.
It still pisses me off that they even bothered to offer such a poorly thought out and poorly tested "upgrade".
I have an older Quad-core 3G Chinese phone that recently started offering me an upgrade to Lollipop - because I no longer need that phone I let it. Unlike the Nexus it works perfectly, faster than before and is a pleasure to use. A curse upon your houses Google and Asus.
Anyway, its dead easy to do - let me know how you get on.
Oh, how's your Honda going? And has your brother made progress on your old R65?