Thanks guys! She's really well, seems none the worse for wear (although she's a bit quieter than normal, but I suppose that's to be expected really).
The x-rays showed bony changes around the pelvis / lower spine area, just where the nerves emerge from the spinal column. The vet said that Darklis is undoubtedly in some degree of pain in that area, but it was hard for her to say just how much pain. We settled on an agreement that it wasn't likely to be intense pain, as Darklis' appetite is as good as the other piggies and her mobility isn't impaired at all.
There are also degenerative changes to her knees (stifles), which were really unusual to see in a piggy of her age (only about 8/9 months). Apparently, that can often be caused by a deficiency in vitamin C, but they could see by inspecting the rest of Darklis' bones (especially her ribs) that this wasn't the case (between the piggies, they eat about £30 of veggies a week, so I should hope they aren't lacking in vitamin C!) It was thought that the knee/stifle problem was caused by her poor posture (probably caused by the long-term low level back/pelvis pain).
So, the treatment is to start her on 0.25mg of Metacam daily, with a view to reducing this to the lowest level possible eventually. I'm to keep an eye on her barbering/fussing with her lower back, and if after about a week, it seems like the Metacam isn't helping her, we'll try another anti-inflammatory (can't remember the name.....something "bepanthen"?).
If that second treatment doesn't work, then the vet said we could go onto opiates.
As I'm typing this, Darklis has her head in the food bowl up to her ears, nomming away on pellets and Redigrass, which is a good sign! I do love my special little girl! x)
Thanks for all the good wishes, Tracey x