I keep getting a voltage surge (well I assume this is what it is). The wipers will go faster, the fan runs faster, the headlights look brighter (and they blown in the past as well). It's quite random though, happens only for a couple of minutes in say a 20 minute trip.

Any ideas?

Could be the voltage regulator on the alternator

Seconded. If left untreated, this will sooner or later destroy the engine ECU, but probably not until it's given you warnings like shutting stuff down and cooking the battery.

Right, voltage regulator it is then. New one ordered this morning from Jorily. Thanks people.

This happened to me and it was the alternator, blew my lights and my radio!

Changed the voltage regulator at the weekend and that problem seems to have gone away.

However, it didn't solve a couple of other issues. The airbag warning light is on for no obvious reason. And the car has a habit of revving uncontrollably on occasions (and sometimes accelerates by itself - thankfully my brakes are ok). I hoped this might be a related electrical problem but seemingly not.

It's a garage job now I think.

Damage to the airbag/airbag warning system is common when the regulator goes, and is left untreated for a while; it may just need a reset.

Ditto the ecu, or the high voltages may have caused some damage.

Hopefully there is someone local to you with the right diagnostic tools to do a reset/system check. Or visit your nearest Skoda dealer.