Hi mandy
I think currys have almost done everything correctly apart from not correctly advising you of the situation when you phoned. The TV was originally purchased under an agreement with your brother, so everything will be in his name. It does not matter that you have taken over paying the WH cover, as you did not change the name on the actual contract, only the payment.
Is there any problem getting your brother to send them a letter with his signature assigning the replacement voucher to you? Ask them if they will accept a fax if this speeds matters up.
In regard to the WH cover offered on the day of purchase, were you told that it was compulsory to take out this cover? Or was it a positive sales offer, that they could offer peace of mind WH cover , but only on the day of purchase. Shops do prey on the 'high' customers feel when they spend money on a desired purchase. I have signed up to cover before in this situation but have cancelled within the cooling off period after further consideration.
Regarding replacement spec, currys will hold a list of which TV is equivalent. You don't have to go with their recommendation, you can purchase any other and use the £699 voucher towards this.
I personally rate the Panasonic Viera range and note that Currys have a 42" for £699 or if you want a 1080p spec, they have 1 on offer for £799. With currys if you find that another company offers a better deal for a stocked, item they will price match, if model the same.
Huckster