I bought my car on 1st March from a non ford car dealer in the south. I was given a 6 month warranty but it does not cover the battery. The battery has been losing power. I have spoken with Fords and I need a battery which is a stop start 775 battery my car has a 770 battery on. Ford want over £258 for a new battery. Do I have any come back with the dealer for selling me a car with the wrong battery. I don't have much money since spending so much buying the car. Thank you for all advice received.

If they don’t cover the battery I doubt you’ll get much back from them sadly. But a few stern-pestering E-mails may get you somewhere. 
 

but if you can’t get anything out of them you could always replace it yourself - just had a quick google and I can see a few AGM batteries comfortably under £150. They’re not hard to replace and there will likely be a YouTube tutorial somewhere, but it may need a BMS reset afterwards

Be sure to check whether you need an EFB or an AGM battery. The car's charge settings are different for each type and you should replace the battery with the correct type for the car.

1 hour ago, An70dy said:

The battery has been losing power. I have spoken with Fords and I need a battery which is a stop start 775 battery my car has a 770 battery on.

5CCA won't make any difference.  It's less than 1% of the overall CCA.

The warranty might not cover the battery but your consumer rights do. I’d be arguing with the seller that the battery has not lasted a reasonable amount of time and was clearly starting to fail at the point of purchase. 
Also what model & engine Ford is this for? Vignale is just a trim level.