You can also pick up the shadow underwood clone cheap, sometimes on Ebay uk, and on Ebay.de also.I bought one not long ago for about £15 unused.
Good pickup for the dosh, picks up the slap ok, depending how It's EQ'd.
I use an underwood, with a pintech drum trigger or Schaller Oyster for the slap. I mix It with a D-tar Solstice(pricey but the best mixer/pre amp I've used and I've tried most).
Stevie, about the Silver slaps, I've got them on one of my basses just now, If you want to try them before you buy give me a shout, and you can try them on my bass sometime.