BMW approved cars of similar age/mileage going for £21k - with discounts, warranty and Part Exchange yours will be worth £2k less in my opinion.
But yours is a very high spec in a desirable colour combo, £20k might be about right.
2008 E90 335i LCI M Sport / Manual / Black / Red Leather / Avon ZZ5 / VMR710
Mods: JB4 G5 ISO / BMS Back End Flash / BMS DCI / AR Design DPs / ETS FMIC / Meth Injection / Spec Stage 2+ Clutch / BMS OCC / Cool Carbon Brake Pads / RB PCV Valve / Forge DVs / M3 Control Arms / Bilstein B4s / Eibach Pro Springs / Eisenmann Race
Wish List: LSD