riding my bike and going to the gym.....long rides
Your comments above could lead to the cause of the problems. I am an avid road cyclist and remember the elbow issues when I first began the long rides (Often to much load on the arms due to poor bike setup and fatigue in the torso). Also the bike setup is very important .

As for the gym there are also a number of exercises that can cause pseudo Wikipedia reference-linktennis elbow. Especially barbell work and triceps pushdowns with a flat bar. Perhaps the most important consideration here though is the neck/upper trapezius area. This needs more investigation and a treatment approach. All of your activities impact on that area. Unfortunately gardening might make it all worse.

Seems you need a more thorough assessment. You should not be diagnosed with 'Tennis Elbow' without due cause. This is afterall a chronic condition i.e. it takes a long time to develop and even longer to become painful. Maybe another opinion is worth investigating