Worcester 30Si replace the drain valve

[rarara]

Got a slow drip when the boiler is on, and looking underneath is suggesting there is a weep from the drain valve. I figure it should be straightforward to replace , but before I take the case off a few questions....

1) can I drain the boiler only, by simply sealing the flow and return isolators?
2) if so, how does the drain valve work - just turn anticlockwise and it starts flowing?
3) given that it's a bank holiday and a new valve might not arrive for a while, is it worthwhile trying a clean and re-lubrication on the existing part seals?

Thanks