I use cheap sports dry bags I got in the perpetual sale at Sports direct.
Made by Karimor I have 1 x large (blue), 3 x medium (green) and one extra small (orange). The larger ones carry all my stuff for a week away and fit nicely in the panniers and the small one in the top box with stuff needed during the day. I also carried a large soft MX gear hold-all that flat packs to nothing and use this to carry all the bags into the B&B's in the evening. I think I paid about £20 total for all of them.
Sounds fiddly but works a treat and cheaper than the price of fitted pannier liners which also seemed to take up a lot of he available space.