I have found that yoga has really helped to balance out the strengths and weaknesses in my whole body. There are a lot of stretching postures that deal directly with the hip flexors. Flexibility and maximising your range of movement helps to build up the muscles that your physio has pointed out as being weakened. Strength and flexibility are easy to work up, it just takes daily practice.
The one stretch that will help you straight aways that i can think of is to begin by sitting calmly and relaxed on the floor with your legs crossed like you used to do in preschool. Breathe in and out a few times, deeply.. close your eyes... then lean forwards gently without reaching out your arms, just let your arms dangle. As you work further into the stretch lean your body further and further forwards loosening up the big muscles that run down your lower back and into the hip joints. When you have reached your limit take another deep breath and stretch just a little bit further, this time reaching your arms out forwards too placing your hands on the floor in front of your crossed legs. Then as you breathe in walk your hands to one side and hang there for a few breaths.. then to the other side. This helps to isolate a different set of muscles in the hip flexors, letting one side rest a little while the other gets a workout. Pretty easy hey! No not really, if you wanna challenge yourself more start the whole stretch from the beginning... this time with your legs crossed the opposite ways.. Yeah.. that burns now don't it!
Each one of us is different with which side is the more flexible, more manoeuvrable, with time and effort you can balance things out in your body
This one stretch routine will at first startle you with how unflexible you actually are, especially if you've been riding bikes for many years. You may not be able to lean forwards at all over your crossed legs when you first begin, but in a couple of weeks you will see a remarkable improvement. It really helps to make your whole hip joint that more moveable, it'll make you like a giant spring again, ready for all those quick movements up and over the frame of your bike.