Well, finnerz, you were right.
Heat wouldn't shift it, whacking it with ever larger hammers wouldn't shift it so in the end had to take a grinder to it and cut the relay arm to bits. After an epic 3 hour battle I had it in many, many bits on the garage floor.
New bearings all round in a 2nd hand relay arm and finally, it's back on the road with a fresh MOT.