So I had a think and decided to take off the burnt side of the ribbon cable and hard wire it to the PCB. I left the other end with the connector intact so that the PCB can still be removed by someone in future with relative ease compared to doing a total hard wire job.
The two pads carrying the filament AC were pretty bust up. Like a restaurant after Steven Seagal has popped off and put all the waiters through the decor. The bits of wire therefore loop through the pads and onto the next solder points for stability then it was all cleaned and checked out.
Here is a view some the other side.
And here it is all put back in the amp. The amp now runs with a consistent filament supply. All it needs now is new power valves and a bias check and away we go!