Martyn, I take it it's the one which is down (wide open) in the photo? If so, is the rocker on it because it's just resting there, or is the rocker jammed and holding the valve open? The rocker shaft looks corroded further along so if you haven't already it would be worth checking the rocker is free.
If you want to remove the valve and lap in properly it's a head off job, but it may be worth checking as Caddy says that it's not just gummed in with light rust & gunge. It doesn't take much to jam things very tightly. If you soak it for a while it will also do less damage when you do remove it.
If it's corroded in, which I assume to be the case, you'd also be looking for damage to the valve stem, guide and seat, or the rocker and shaft if the rocker's holding it down.
If it's bent because of a mechanical malady which has resulted in the valve hitting the piston due to a timing or some other failure you need to check the bottom end as well (piston, rod, big end, crank, timing gear)