Sudden swelling of feets/hands/face can be a sign of pre-eclampsia so speak to your midwife, just in case.
The swelling is caused by water retention, which is common in late pregnancy. I had this last time around from 36 weeks, but it was the height of summer and heat can make it worse.
To relieve it try:
Putting your feet up whenever possible, to help your circulation. At work, keep a footstool or box under your desk. At home, try to lie down, on your left side, as this takes pressure off the inferior vena cava.
Put on support tights before you get out of bed in the morning, so blood has no chance to pool around your ankles. If they don't help, see your doctor. She can prescribe compression stockings.
Drink plenty of water. Surprisingly, keeping hydrated helps your body hoard less water.
Exercise regularly, especially by walking, swimming or using an exercise bike.
If you're travelling long distances, take regular breaks to stretch your legs.
Eat well, and try not to have high-sodium and salty foods, such as olives and salted nuts.
If your skin isn't too tight and painful, ask someone to massage your ankles and feet, upward towards your knees. This can help to move the fluid away from your feet.