I have just been through this exact situation (with Elephant.co.uk) in May. So I speak with some experience.
Basically the FSO recommends that insurance companies should pay out Glasses Guide retail value, minus any excess, in the event of a write off (and that was actually written in the small print of my insurance documentation). Approach a friendly motor dealer and they should tell you what Glasses retail price is for your car. That's what you should be aiming for. Write off to the insurance and tell them that's what you want.
If they still refuse to raise their offer, then go onto the Auto Trader website and print off some ads for similar cars. You'll probably find that they're priced around Glasses Guide price anyway - since that's what the trade uses to price cars. Send these off to your insurers, pointing out that that's what it's going to cost to replace, and ask that they increase their offer.
If they still refuse, then after their final offer you can then refer the case to the FSO.
Never accept the first offer as final settlement (still bank the cheque though). I eventually got a reasonable payout in 3 separate installments.

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Be in no doubt: Insurance companies will try to "Do" you in any way they can. I'm disgusted at the way they try to evade their liabilities.