I wouldn't trust a silver mica at that voltage unless it said so on the tin!
Try this:
http://cpc.farnell.com/lcr-component...20This%20Range
I've used these "LCR" products before, and I reckon they work perfectly in radio restoration. This one is polystyrene, larger values tend to be polypropelyne, but the fact remains that I've used quite a few of "LCR"'s capacitors to replace old waxies and have always been happy with them.