I have saved with Park hampers for a few years now, I have had no trouble with them and The money is ringfenced by the government so it is not released to them until the vouchers are sent...... I tried to save up and found it easy to just use the money and leave an IOU in it's place!!!!!
I pay £15 a week and get £780 a year for the kids it makes a Huge difference...
I get to spend them at most major high st shops, Boots, Argos, Debenhams, Matalan, Toys R Us etc.........