I finally got the steam valve open today. After the boiler, it was by far the most difficult fitting to get open - I think the washer / gasket fused hard and practically welded the lot together. I'd tried heating, and a big spanner but nothing worked. Went round to a friends house with a decent vice, and with a HUGE spanner for more leverage and it finally came free. A bit of a green tint inside, but really not too much scale. So here's a couple of pics after decscaling and a good scrub / polish with one of those green / yellow pot scrubbing sponges. Interesting how the valve works. The shaft has an off centre cam that turns right to leave the outlet closed, and left to move a spring loaded pushrod and open. A tiny screw inside is the stop for the cam & collar on each side. 
And after much pontificating, I couldn't resist dropping the other worn / corroded shaft from the three way valve, into a welding-machining workshop to try and get it repaired. I'll find out in a few days if they managed to work a miracle (or not...)
