Depends on the version...
There are two Pentax-M 28mm f2.8s, the earlier one has a chrome ring on the front (like the other M lenses) and isn't considered particularly good. The later model doesn't have a chrome ring and looks suspiciously similar to the Pentax-A version aside from the aperture ring not including the "A" setting.
Personally I'd still hold out for a Pentax-M 28mm f3.5, you won't notice the half stop difference and the image quality is much higher. I managed to find a nice one for £35 or so a while back, they are out there so keep looking. The 28mm primes are holding their value as they make superb standard lenses on the Pentax DSLRs, and with Pentax new prices having been hiked recently people are looking for used kit.
Assuming that's the Pentax-M 135mm f3.5 I'd keep looking, you should be able to find one for less with a bit of hunting. Can't remember if I suggested earlier but try a daily ebay search for "Pentax" showing newly-listed items first and only showing Buy It Now listings, that's about the only way to find real bargains now.
Pentax made loads of 28mm and 135mm primes as they were the standard kit (along with the 50mm sold with the cameras). There's also a 75-150mm f4 zoom which is pretty compact and has excellent image quality, London Camera Exchange have one listed for £19.99 at their Worcester branch at the moment. Might be worth a phone call?