I gave it another go after reading good things about Lineage 14.1 (Android 7). Flashed it OK, flashed Gapps OK, installed Magisk, installed Magisk Manager, installed the banking app - failure. It spotted the non-standard ROM. It surprises me that custom ROMs appear to be so popular given how difficult it is to use normal apps on them. A new phone for me, definitely. Thanks, guys, for all your help.