In reply to Mike Nolan:
If you buy them both on-line, you are legally allowed to return them within a week for a full refund. So you could by them both from an e-retailer, try both on, and return the one that doesn't fit. Or return them both and try something else.
Osprey do good packs if you have the budget.
Alternatively, buy a cheap-but-good-enough pack for DofE, and then buy a different pack for when you go lighter. For the price of a POD Black Ice, you can get two or even three rucksacks that are OK.
Tesco have an acceptable range of rucksacks. Look at some of the cheaper stuff in Blacks and Millets.