I have a dilema..it's called cash...
If you have panniers that are structurally sound, then all you need is a some liners to keep your stuff dry Don't worry about putting anything on the outside of the panniers; you're not trying to keep the pannier fabric dry!
Bin liners are too frail, rubble bags too thick, but you can get 250g clear bags of up to 2ft x 3ft that are perfect. There's lots of suppliers on the internet, and even the wholesalers will send you samples if you give them a bit of chat.
I'm sure Ortliebs are fine panniers, (although the flap tops don't have that extended tunnel entrance of the rollers).
But use what you've got. Tour NOW! You can pimp your ride when you're dead.
(25 year old karrimor panniers still keeping everything perfectly dry with the help of 5 quids worth of liners.)