I'd like to suggest that if you plan to use the Heathrow Express in the next few days, you check carefully when and how your service will operate.
The entire fleet has been taken off due to a structural crack found on one of the trains. How long it will take to inspect and repair all those trains I do not know.
In the interim the services will be using Heathrow Connect trains. I don't know what that will do to the schedule or overcrowding, nor do I know how the Heathrow Connect timetable will be serviced.