Hi again Gilly, I did understand the first time that your pain was only when walking. I remember that when I was pregnant, just as for many other women on this thread, cycling was more comfortable than walking - just because I was more weightless on the bike (at 13.5 stone?! Weightless! Ha!).
However, unlike Pidge, you do have this uneven leg length issue which cannot be ignored. Even if it's not the cause of your pelvic pain, cycling may possibly exacerbate your pre- and post-birth leg, back and pelvic problems. When you cycle your muscles become shortened (from the hips down to the ankles) with the repetitive movement. That's why it's so important to do good leg stretches afterwards. Bearing in mind that your pelvis has opened up to accommodate your growing baby, giving you a lovely Daisy Duck gait when you walk, the repetitive cycling motion will encourage your lower body muscles to stay in this position even after the birth. That's what happened to me.
I gave you the example of my small bike frame to indicate how any imbalance in alignment, when combined with repeated movements and pregnant posture, can cause subsequent problems. In my case I ended up with a bizarre 'warming' sensation in one of my ankles, calves so tight you could crack nuts with them and a lower back that ached every single time I sat down. It took 18 months for the back to sort itself out, and for the rest I went to a specialist.
So you do need to think carefully. You only have a few weeks left before the baby is due. Don't forget that women at your stage of pregnancy used to be prescribed bed rest as a matter of course. Is it possible for you to be a bit less SuperWoman and a bit more PamperWoman? I'm assuming rightly or wrongly that this is your first pregnancy, in which case you will never again have the time and space to look after yourself like you do now! Could you get your DP to drive you to the gym, or could you take a taxi once in a while?
Finally, what advice did your ob physio give about cycling in your situation? If none, perhaps you should call them for a medical perspective.