I am back and with success!
I wouldn't undertake this unless your HD25-1's are completely shot in the connector department. I am an OKish solderer and only do it for necessity, mostly remote control cars/planes.
If you are an good solderer please don't look at the solder picture, it is a disgrace. The other one was slightly better but it is terrible as well. On the plus side it all works and no one will ever see it again...
It was tough soldering those little wires and I had to solder each one a few times as either the voice coil wire would snap or the solder would give. It was much easier working with the AluSol than regular solder but it was brittle.
I decided the best course of action was to completely gut the current connecting mechanism. I pulled out the spring and brass posts. Then drilled a bigger hole to put through the headphone cable and removed the inner plastic separator.
I split the cable and used a mini butane torch to burn off the insulation on the wire strands. Be very careful of doing this on the aluminium voice coil wires as more than a fraction of a second and they will disintegrate. I found that the red insulation on the voice coil wires actually comes off in the soldering process.
I tinned the speaker cable wires and then set the voice coil wires to be pushing against the solder blob on the speaker wire with natural spring.
Then simply melting the solder blob had the voice coil wire sink in to it.
I put a cable tie around the end of the speaker wire so it wouldn't get pulled though the hole in the casing.
Then it was just a matter of fitting it all back together.
How do they sound? Great but it has been so long since I listened to them I can't tell any difference and in the mean time I picked up some Beyerdynamic DT1350 to console me in case it didn't work.
Now I have two fantastic sets of portable headphones :)
Thanks everyone for the help and suggestions!