Britannia Main Beam stuck on

[herrbartlet]

New (recent) electrical problem:
Side lights & headlights (low beam) working correctly from dash switch.
With headlamp switch on or off, using stalk to 'flash', the main beam will stay on after stalk released.
When on low beam, pushing stalk forward will engage main beam but pulling it backwards - they stay on 'main'.
Switching off headlights or switching off engine does not cut power to main beams.
Checked stalk with circuit tester. Flash and main beam terminals seem to work.
My inexperienced electrical knowledge leads me to think: faulty relay.
603 wiring diagram shows one 22/26RA type relay in the circuit. However there are mostly 1" cube (28RA) 30/85/86/87 (5 pin) relays around the battery and removing one of these does cut the power to the main beam. Reinstalling said relay allows me to use the sidelights & headlights (on low beam only). If I accidentally engage main beam, the only way to cut the power is to pull out/reinstall this relay.
One would think swapping this relay for a new one would cure the problem, but it does not.
It leads me to believe there is a second faulty relay but unsure if I should be looking for another 28RA or the larger 26RA or where it might be located. Any suggestions welcome...