Well after simmering the timing cover in saturated solution for hours all I've got is a stained cover and a baking tray full of alum crystals.
I'm not sure why it hasn't worked although I think the hammer screws may be stainless rather than the case hardened steel internet experts would have me think. Of course there is always the possibility that the whole 'remove steel parts from aluminium using alum' thing is just a load of internet bollocks.
For Plan B I've bought a 1.5mm carbide end mill. I just need to figure out a way of mounting the cover on my lathe or pillar drill securely enough to mill out the screws.
Of course there is always Plan C, which is to drill holes to the slightly to the side of the two sets already there and Plan D which involves araldite.