I believe that's an incarnation of the kit lens that was supplied with Pentax's autofocus film cameras. Don't expect any better pictures than from your 18-55 but if the extra reach is useful, go for it.
After all, the Pentax DA 18-55 (and the II, the DAL, the WR etc etc, they're all the same) is the best kit lens out there.
Remember that you will have a startling loss of wide-angle. Try setting your kit lens to 35mm to give you an idea of where it'll start.
To be honest, if I had £20 and your stuff, I wouldn't bother. I'd see if I could get hold of a Pentax-M SMC 50mm f1.4 or f1.7 prime lens, and you can teach yourself about manual focus and stop-down metering while taking some cracking shots in the process. Spend £45 and you could get the Pentax-A instead (no need for stop-down metering), or £150 and get the modern autofocus (the SMCP-FA version).