Battery telltale

[P@ulW]

After about 50 miles' driving the battery telltale lights up. This has happened the last three times I've driven the car any distance, but it doesn't happen on shorter trips. A friend put a volt meter across the battery (which is about three years old) the other day, and the reading was 12.3V. The alternator's less than a year old and the dashboard voltmeter reads a steady 13.5 when driving. The first time the battery telltale came on, I think the voltmeter spiked rather higher for a ten minutes or so. I've got to drive the car from Andalucia to England next month... Should I be concerned and what should I do? I'm no mechanic, and my Spanish is rudimentary...

Thanks in advance for any suggestions.