Horsey and missmummy, I was referred for the GTT because baby's abdominal circumference continues to measure quite large (around or above the 95th percentile) in scans, and this can be an indication of gestational diabetes.
My bloodwork at my 28wk MW appt came back fine, though (they look at glucose levels then), and all of DS's other measurements are normal/average, so I was hopeful that he just has an appetite to rival everyone else's in this family of eaters.
BTW, I just turned 40 and am at UCLH. They don't routinely give the test to all older mums there, but maybe other hospitals are different.