UPDATE
Having spent agaes on the phone to BA whilst the guy went off and checked, he discovered the £1179 (876+303) price is not correct and the correct price is £1266 seen on the Price Quote page. He is going to have this sorted out so that nobody else is as confused as me.
The difference between my fare and the £1266 is down to the F MFU on the Outbound and the CW on the Inbound