Be prepared, London tube trains do not run 24 hours!
During the week, trains start around 5am from the ends of the lines so reach more central parts around 5.30-6am. On Sundays, tubes start around 7am, but check in advance for Weekend Engineering Works.
Tube stop each day around 11.30pm-12.30am.
When the tube stops, don't give up hope of getting back to your hotel. Public transport is still available in the form of night buses. Night bus routes are similar to the daytime bus routes, but have an 'N' prefix. It's always best to check with your hotel reception which night buses go nearby.
If you can't find a suitable night bus, black cabs are your next best bet. They do have to charge a little more at night time, but heh, we don't all want to work at 3am, do we? Black cabs are licensed and therefore a safe option. Do not take an unlicensed minicab in the middle of the night.