Ok, I've finished writing up a quick .doc file. Let me know who/where to post it to.
A much abridged version:
Cut the track at point "11", circled in red.
Then solder a bridge between the first and third (circled in green) pins on that side of the chip, as seen in a pic further down.
Cut out the back of the perspex holder so the new LED fits in all the way through to the board:
Then using a bit of prototype board as a guide, drill out 10 holes for the LED legs in the centre of the perspex guide
Also drill out the little pads that used to carry the signal to the LCD.
With all the holes drilled, it should look like this:
I snapped a drill bit in one of the holes.
Then turn it over and with the bit of prototype board, connect the right LED legs up to suitable resistors in the right holes for the signal:
Not particularly neat underneath but does fit in the stock location and works.
Regarding colour of LED, I was hoping to use a black/white one but finding anything in the right size other than red proved difficult. It's not so bright and you have some control over the brightness by which resistor you choose. In the end, I'm happy with red, it ties in nicely with the other red dash lights.
The cost of the components is small and I ended up ordering multiples of the bits I needed so if anyone has any dead boards they don't mind donating or selling, I might try to do a batch to use up the bits I have.