I've sent the complete cutting assembly away to be professionally disassembled, cleaned and sharpened. I'll let you know how I get on.
Meanwhile I went to see the other mower this morning. It's a 14" SB45 fitted with a Kirby 2HP engine and for not much money brought it home. With the old cars and now old mowers, my good wife is starting to shake her head

I've started a new thread (with pictures) for the other mower.
Chris