This is my small contribution to Geekzone diy networkers out there on a budget (maybe someone will find this info useful one day). I found snip its of references to this process already around (some on Geekzone) but no one really reporting success or linking to the following useful specific pages .
After a little research I upgraded the firmware of an old Vodafone (HG556a) ADSL modem router to OpenWrt using this page:
Then using a laptop cabled to it via ethernet I could configure it quite easily using this basic guide to dumb it down and turn off most things except LAN and wifi, using this link:
I had previously installed a Ubiquiti Unifi AP AC LITE which is great, but a little wasted in my garage/workshop. Now I can redeploy the Ubiquiti in a more used location.
Honestly I was surprised the openwrt software worked so well and the process was quite straightforward. One small step for a man...