Just thought I'd add my experiences, based on some of the above suggestions.
I lost my first piggy at the age of 2 years to a bladder stone that was wedged in his urethra. The vet (not cavy-savy, I'm afraid), attempted to remove the stone, but failed. He died 2 days later from post-op complications. I was not given any pain medication or gut stimulants, and was never advised to hand-feed. I didn't know about the forum then, and had only very basic piggy knowledge. I assumed the vet had given me everything I needed to look after him, so never thought to question it. I blame myself for not doing more. I vowed to find out everything I could about piggies from then on, particularly bladder issues. I joined the forum, read everything I could, and changed vets.
Benny was kept on woodshavings. He had tap water, and 4 months before his death we moved to a hard water area. His veg was washed in this tap water and given to him while still wet. He was fed Pets at Home nuggets. His diet was varied, but perhaps contained a little too much kale. He was a small boar, and weighed around 1kg.
His cagemate, Jerry, developed signs of bladder sludge 3 months after Benny's death. By this point I had done my research and had changed to Oxbow nuggets (timothy-based), timothy hay, and filtered the tap water. Kale was cut out altogether. Jerry has now lived with bladder sludge and cystitis for 2 and a half years. He had his bladder flushed in June 2012. His symptoms are managed with a low calcium diet, a low daily dose of Metacam (which is increased for a short period if he seems uncomfortable), a capsule of Cystophan, and a pinch of Phenyoxybenzamine. He weighs 1.1kg and is now on vetbed. His veg is washed in filtered water and given to him while still wet. I am currently trialing Jerry with some new nuggets specifically designed for bladder piggies - Supreme Science Vetcare Plus Multi-Modal Formula.
Perhaps Jerry's bladder problems are a result of my earlier naivety, but it seems strange that his symptoms only developed 3 months after I had made the changes.