OK, if anybody's interested, the dashboard backlight is NOT fixable.
It appears that the "backlight" is actually an illumination strip - i guess it is some or other type of flourescence activated by electricity.
Getting the unit apart is another beast, which presented a nice challenge. The front cover is glued tightly shut so cannot be lifted out. So I drilled it out, using a small bit and about 300 holes (just a guess). After that it was plain sailing. I took everything apart except the needle because i was afraid that once the needle comes off it might not fit properly again. The strip was then almost accessible - just had to cut it so that I could slip it out around the needle.
My plan is to use 5 or 6 hi-bright green LED replacements. Now that the illumination strip is out I can get the LEDS right up against the perspex reflector lens - at least I'll be able to see my speedo now at night! The front cover will be reglued / resealed into place with some good silicone.
cheers