Hi,
Yep, everyone will need a chest x ray. Its part of the requirements for the issue of an initial class 1 medial (see jonny connors post for the link). When i booked my medical they asked me to email them my home address and telephone number etc, so that they could forward me the paperwork required to get the x ray done at my local hospital. If they ahvent asked you to do the same, best call the med dept at heathrow again and ask if they need an email, or if you tell them over the phone.
Have fun!
C