Peter, I'm also auditioning mid-price speakers at the moment, so my thoughts so far, if they're of use...
Quad 12Ls (circa £500)- a good speaker, although not the fastest sounding. I felt it was a bit 'boxy' in sound with an unwelcome 'clompiness' effect at times. Has been well-reviewed although failed to get a Best Buy in HFC a few months ago. This is not a floorstander but is as attractive a (standmount) speaker as you'll find for the money with it's lacquered cabinet.
Rega R3s (£450)- Fast-sounding, exciting, detailed and musically coherent: good for poppy electronic music, indie-pop etc. However, there is no low bass that I could find in this small floorstander, so some may find the whole lacks body and warmth; realism could possibly be improved too. If you audition these, do check that they have been properly run-in to evaluate their true character. They've just won HFC Magazine's best sub-£500 speaker award.
Quad 21Ls £750- In contrast to the R3s the 21Ls sound laid-back, can be biwired and are (reportedly) fussy to position, preferring free space to close wall proximity. These sound detailed, smooth, safe and even; particularly beguiling and involving with slower, ambient material. Punk and hip-hop sound rather too well-behaved through these.
I'm also struggling to find many £500 floorstanders; it might be worth you seeing if Spendor do some, although I personally have no experience of them.
Good luck to the two of us: Planar 3-users both!