If I search Autotrader on your 1.8 3dr model and year it brings up 6 cars. Knock out the daft dealer one at £5,995 and the cheapy at £2,495 and you are left with 4 cars between £3,995 and £4,995.
That puts your payout offer of £4500 bang in the middle and in truth I think pretty generous.
But... I guess you are expecting more as it's a "Black Edition"?? To be honest the extra value of that is a very personal thing and going to be hard to value over a standard car. However as well as the (to me) over priced £5-6000 ones of your year on Gumtree there is one under £4000 and one under £3000 of that spec.
So there is actually loads of data to start negotiating price. Bear in mind you should not expect to get a retail value, nearer a trade or private sale price.
A possible second option could be to buy your car back and have it properly repaired depending if it was a total... or just an economic decision to write off which is pretty common.