If your policy provides new for old cover then your insurers can replace the item or offer you funds/vouchers at their discretion. I do not think you can insist on the same make unless there is some particular reason e.g. compatibilitywith other equipment etc. They must, however, offer you either an item with at least the same functions or better (if better they may ask you for a contribution for the betterment) - otherwise they must offer you sufficient to purchase an item with the same functions.
Ask them where they have got the like for like quote that came out at £400.00. They dont have to give you £1250 if technology has moved on and the items in the market place now retail more cheaply than they did when you bought your original lap top.
If you cannot resolve this then your 1st step is to write a recorded delivery of complaint to your insurer. If they do not respond in a reasonable and timely manner you can then escalate this to the insurance ombudsman.