Still haven't heard back from them yet, (neither has the recruitment dept at NATS,) but I think they would tell you there and then if something was wrong (like I may have to have another ECG because the one I took was "fast").
They don't have any x-ray facilities at Heathrow, hence the "get your own x-ray" stance. It's a bit inconvenient to make the arrangements (see your GP, get referred, get an appointment with x-ray etc etc) but easy enough.
Best of luck to you