i would imagine it's the same over there, but in the US (midwest specifically, where we have seasons), the best time to buy a convertible is fall/winter.
however, i have to put in my vote for a honda s2000! highly reliable, extremely fun to drive, cheap (relative to merc, bmw, porsche, etc), will keep ~50% after 5 years, fastest drop top at <6sec, ...the list is endless.
I've had my Honda S2000 just over a month now.
Frickin' amazing animal of a car!
Like driving a high powered motorbike with 4 wheels
A totally enjoyable and fun ride, despite spending most of my travelling time in gridlock :x