initially most would go straight for the Actuator, which are costly at circa 1,500 sterling...have a look to see whether
you have a faulty wiring loom,
has it been re-calibrated recently ?
could be a faulty selespeed pump
the battery should be reading 13-14 Volts D/C when the engine is running, if you get 14v turn on the main beam headlights it should not drop by more than 0.2v, if it does, then either your alternator or wiring (probably the earth) is knackered..
after the turning the engine off the voltage should not drop below 12.5v, if it does, with no load, it probably needs to be replaced...
have a look through the search option above, it will have some invaluable help...
all the best