In reply to Piers: I am a big fan of the exped dry bags they are very good and will keep things dry in most conditions, but are not super hard wearing. With somethings as vital as ur sleeping bag you really want to keep it dry. The mantra I use to keep things dry is a bag in a bag in bag with vital items such as sleeping bag placed within the inner most bag. I know this seems overkill but its has worked for years.
In your situation I would see if u can get a light weight outer cover to protect the dry bag from physical damage or just use a another dry bag.
Hope this is of some help and have a great trip