Vauxhall Tigra Alternator?

I'll tell you what it probably is. The alternator includes diodes and similar that prevent overvoltage and stuff like that. If the diode has blown, you have a voltage problem, e.g. discharging back through the diode when it's off, and that. The alternator may be fine when running, e.g. normal voltage. A host of faults that boil down to a 10 pence diode result in the whole 50 quid alternator being replaced.

As your description covers "while running", it may be over voltage, or as a possible alternative, it may be some major issues with the wiring loom. Looms can short out all over the shop as well. And this can cause the diodes, etc, to blow out. So replacing the alternator may not solve the underlying problem, although it may.

This is a recurring theme, by the way, nobody seems to diagnose cause any more, they only determine how many large chunks of the machine to replace, there's a lot of motherboards with duff capacitors which have been thrown for the cost of a soldered in capacitor, 15 pence and a minute with a steady hand and a soldering iron. This becomes far more true of cars with EMS and similar, the garage won't try to find out what's really broken when replacing the whole engine control computer will solve it, and cost them nothing more than your wallet. Cynical, I realise.

Try to find an auto-electrics chap near you. They often live in grease-coated hovels full of ancient alternators and starter motors and they'll fix broken ones really cheaply - £12 for my last one.

Si