http://www.tfl.gov.uk/tfl/livetravelnews/realtime/tube/default.html shows the situation as it is now, but by setting Future Dates to the 21st April you can see what they have planned for us.
At present, there are no engineering works planned for the east of London. It looks like the DLR is okay to Custom House
(Excel Centre) but one can never be 100%... so check the website again before the show!
Also, for those of you coming in for the weekend
remember that London has the Marathon on Sunday, 22nd April... http://www.tfl.gov.uk/tfl/gettingaround/londonmarathon/