hi i have skoda octavia tdi 51 reg boot light has never worked , fuse ok and changed bulb still does not work , any ideas

They are negatively switched, so have you checked you have a perm +12v to the light fitting?

That should indicate if there is a break in the wiring from the fuse box at the front.

Does the door open symbol on the dash light up when the boot is open?

no

I would suspect the microswitch in the lock.

If you unlock the car then open and close only the boot what happens? Does the car relock itself?

no

If you unlock the car and don't do anything does it relock?

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I would suspect the microswitch in the lock.

If you unlock the car then open and close only the boot what happens? Does the car relock itself?

I think it is also the boot microswitch.

As the boot light is not linked to the doors opening or interior light the opening of boot sets the light.

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I'd start by looking at the microswitch. If that checks out okay, there is a connector down by the CD changer, slightly behind and below it which carries the power for this light (or maybe the switched earth, I never did bother checking which...). It is certainly not uncommon for that connector to be corroded and make a slightly bad contact. Mine used to flicker whenever disturbing the connector, until I gave it a severe going over with some deoxit DN5. I think it is the middle of the three connectors, and they are a right PITA to seperate, let alone get to without taking half the trim apart. Exhaust all other avenues before going down this route.