It is step free...but do not follow the transfer signs. Instead head to the exit from the Piccadilly Line and when you reach the main ticket hall (with all the ticket barriers) simply follow the signs for the Jubilee. Step free at London Bridge too though you do have a 5 min from there to the hotel. Get on the back of the Piccadilly line train at Heathrow to avoid a walk along the platform at Green Park.