I take it you are on a business trip if you have booked the Docklands hotel no other reason for it, as it not central London and does not have great access to Central London. Best way from Heathrow Airport is the tube to green Park, then jubilee to Canada Water then taxi, uber or walk from there.
Sunday for Gatwick i would probably do a pre-book taxi you cab get one for about £50 google airport taxis London.
Going back form Lancaster Gate you can take the tube back to Heathrow.

