I would be amazed if its worth anymore than £800

Autotrader has Campus models around that age and mileage for between £2k and £2.5k.

That's not what its worth - that's what sellers think their car is worth. Two very different things. HJ's 'Pricing' section gives a range of figures, from the 'dealership' price to pay (highest, more matching your figure) to PX (lowest). I would say its probably worth somewhere around the £1500 mark that they'd get if they sold it privately, not what you'd have to pay at a dealership to buy it (as that includes their profit margin).

Low mileage won't make that much of a difference on an 8yo old small car (maybe £100 tops) - its mechanical, electrical and structural condition and reliability/FSH that is more important on older cars.

Why is the general advice given to look at autotrader to argue a derisory insurance write off offer then?

Because if your car's being written off, then you want the maximum money you can get. The implications here are that Autotrader prices are inflated.