Rover owners: A little advice please, what do you think its worth?

I have a 2003 Rover 25 1.4 103 il, I've owned it for the last five years and it has been a great car, I am reluctant to part with it but since having our little girl 14 months ago, it's now become to small to carry all the required stuff around, especially when we go on holiday. I've tried in vain to get my wife to travel more lightly...but she insists that she only takes the essentials, I even brought a fold away roof rack for the last holiday, but we were both terrified that the suitcase we put on it on the way there was going to come flying off at any moment and the car did not want to go up hill with it on either.

Because of these size issues, I started looking at ZT-Ts and 75 Estates to replace it however, the gap between what the new car will cost me and what my car is worth seems to be larger than I was hoping for...

Last weekend I went to look at a 54 ZT-T 135, he wanted £3,500 for the car and offered me £500 in part ex for mine, is that about right or do you think I could get more if I sold it privately? Here is a brief outline of the car and some Pics,

* 2003 Rover 25 on 03 plate

* 1.4 103 K Series Engine with just under 62,000 miles on the clock

* Stamped service history up to around 31,000 (2,000 miles before I brought it, since I've had it has been serviced every year by a family friend and have every MOT and paperwork for most things I have brought for it, CV boot, speakers etc)

* Taxed Until End of August

* MOTed till 21st July

* Recent CV Boot and Joint on passenger Side

* New front brake pads fitted a few months ago

* Pollen filter replaced a last month

* New Battery Nov 2012

* Air Con

* Electric Front Windows

* Electric Door Mirrors

* Stereo controls on steering wheel (I've never had them working as it didn't have the original radio in when I brought it)

I know mods don't increase the value of the car but I will list them anyway,

* Half Leather seats from 200vi (Material section in middle of driver seat is starting to come un-stitched and needs repairing)

* TF Mirror

* 15 Wolfrace Alloys

* Sharkfin Spoiler

* Audiobahn speakers front and back, unfortunately they won't be of much use as the head unit is my wife's (I took it out of her old car as it was newer than mine, but now she wants it back lol)

* ZR Trophy Lights

* Xenon headlights

* Front fogs

* GTI Grill

* Pipercross Cone filter with cold air feed from below

* Citreon Xsara mist washer jets

* ZR Gearknob

* ZR heater surround

* Centre rear seat belt is a lap belt from a 45 as the inertia reel one broke when I was changing the seats