Okay... So I had MC 1.4.2 loaded with forge and ~40 mods. When I tried adding Pixelmon, it came up with a lot of conflicting IDs so I just decided to get ID resolver. When I placed it into the Coremods folder, Minecraft started and came up with a black screen straight away.
MC Error log: http://pastebin.com/Bq67q29X
Has anyone got any suggestions?