If you are arriving at Heathrow this Sunday morning (the 24th) or need to get to Heathrow you will have limited train service either way.
Until 11 AM, there will be no Heathrow Express, Elizabeth Line between Paddington and Heathrow. There will also be no Piccadilly Line trains between Kings Cross and Hammersmith. (The Piccadilly Line closure is both Saturday and Sunday.)
You can take the District Line, Circle Line or Hammersmith & City Line to Hammersmith and then change to a Piccadilly Line train.
This is for planned engineering works.
Another Warning....Winter Wonderland opens today which means large crowds around Knightsbridge, Hyde Park Corner and even Green Park stations. With the Piccadilly line closed, the District Line trains will be very crowded.
Have fun.