M25 Speeding

Hi to all
This is my first posting, and hopefully someone can advise me.

I have received a speeding notice for 65MPH from a variable speed camera on the M25 in excess in a 50mph speed limit.

On the 1st overhead gantry it did state a 50MPH speed limit, but on the 2nd gantry the warning signs were blank, the 3rd gantry stated 50mph again.
The automatic camera flash me on the 2nd gantry with no warming speed restriction, I was keeping to the same speed as the other drivers, and I did notice the car in front of me got flashed, but it was too late for me to slow down.
Would I have a case if I contested the fixed penalty notice....I have a camera in the front of my car with evidence of the gantry signs.

I have asked other people, and they say I was in the wrong to increase my speed until the 50mph restriction was clear by the sign.

Thanks

Steve100