I would return them, CM123. In my experience, once they develop a "Bug Bear" about an item of uniform or clothing, it tends to stick. Once you have washed them you can't return them.
We use M&S trousers because they have three different lengths, along with several different styles that seem to come in slightly varying lengths, and the waistbands are adjustable. As long as you get the length right, the rest sorts itself out for the most part.
As a fairly able seamstress, I have to say that letting down trousers always, always leaves a mark. The first hem inevitably gets scuffed and faded, and it shows up when you let them down. On some items it is perhaps tolerable, such as cricket whites, but it will show up quite badly on cords, Tipsy.
Also, Wundaweb is not a permanent solution - it often gives out after a couple of washes/wears.
(Sorry to all those not proficient with a needle and thread!