Trade In prices...how much !

I just want to let off a bit of steam & maybe if I am lucky some of you might make some suggestions, as to how I should progress with the conundrum I find myself facing.

I am the (now proud) owner of a petrol Octavia VRS (it's had it's problems, but (touch wood) we seem to have fixed the worst of them now, so I'd like to keep it for a while at least).

My wife is also the proud owner of a Subaru Impreza (1.5r) Sportwagon, which is only 20 months old with 7.5k on the clock & cost <£13k brand new...her first (& probably last !) brand new car. It was an emotional purchase.

We are now expecting twins in Dec & were looking to trade the Scooby in for a Mazda 5. In order to get a price I asked the local dealer to give me a trade in price against...a brand new 2.0 Diesel Sport with a Lux pack & the same car but 2.0 petrol (both cars would cost in excess of £20k list). I figured after a discount & trading in the Scooby we'd probably need to finance £10k ish....say £17.5-7.5k?

That was Sat...today I've had a message left (which I had to listen to twice, as I thought I'd misheard it the first time !)...offering me £5k against a £21k diesel or £4800 against a £20k petrol...about 37.5% residual after 20 months...needless to say we won't be doing that deal.

I still need to test drive the a 5 & may go down the used/ex-demo route to make it a little more affordable...or even buy thro' an online broker (that makes me a little nervous tho'...any experiences)...and consider flogging the Scooby privately...it's absolutely mint & would make a mega first car for someone or just a handy 4x4 second car...it's gotta be worth £7.5k at least??? If not we might just have to keep it & stick the MPV on hold!