Don't know if you know Century Wharf, but it's probably about a 10 to 15 min walk from the back end of central station (depending on your walking speed) so factor that in if you have literally a half hour.
It's also a gated complex with a lot of buildings so getting in and finding your way around can be fun. I once visited a tourer who knew the name of the building she was in but couldn't describe how to get to it and I couldn't find it on any of the signs. Spent ages wandering round trying not to look suspicious
Hopefully this one will be able to give you clear directions!