Unfortunately backache is very common, especially later in the pregnancy. The worst things for you right now are sitting for long periods of time and standing for long periods of time. Im assuming youre definately doing the first? Is there any way you can limit the hours youre working? If you could just work the mornings and spend the afternoon with your feet up, it may help both the backaches and the swollen ankles. Do you work from home? If you do, another thing you could try and do is alternate resting and working through the day. Do an hours work sat down, then go lay on the sofa for half an hour with your feet raised and a nice hot drink and a snack. Then go do another hours work.
Have you seen your midwife since you last mentioned on here about the swollen ankles? Even though Im on maternity leave now (Im 35 weeks) I still get them and sometimes last for a few days but normally go eventually (until the next day and they come back lol) If you havent already mentioned them, do so when you next see your midwife. There may not be anything she can do about it but she will be best the person on suggesting ways to eleviate it a little. If your hands and face start to swell though, call her straight away.
Just read over you question again, as you spend so much time sat down at work, is there any way you can have something under your desk for you to put your feet on whilst you work? Every now and then take five minutes to excercise your feet and ankles as well. Stay sitting but rotate and flex your feet.