would add Mifsuds to the two listed above - had excellent customer service from them. They parcel up well and post quickly and often have good prices. Their website isnt the best but they are very quick to answer email - useful for overseas purchases I find
SRS is often my first port of call for Pentax, great stock even on obscure stuff and generally great prices, but not always convinced about the standard of their packing (my K10D body in its Pentax box was put straight in a Citilink plastic bag, no padding) and CityLink are a bit rubbish at delivery round here
Tried to use Park on couple of occasions as they have good reputation, but their web site doesnt show stock levels (or it didnt last time I looked) and I have been caught out waiting for dispatch/delivery when they dont have it in stock and didnt let me know - though they were always helpful when I phoned
If you are feeling flush, Robert White stock some lovely things that will be a bargain at 1 Pount = 1 Euro
hope that helps