Had a new battery fitted along with alternator and it only worked for about 20 miles.
On drive home from garage speedo failed and all brake lights came on and spoiler stayed up.
Had to get it towed back to the garage who can't figure it out and say it's not their fault but an electric fault.
Anyone any ideas please?

 

Have you tried another battery and had the alternator checked. I do not believe in coincidences like this.
Sounds like the alternator failed and you were running on just the battery or vies versa.

 

It is a brand new battery and alternator and the garage said they've tried everything I'm no good with cars so wouldn't have a clue

 

Have you tried to check the ground from the battery to the frame? Many members have added a separate ground from the battery to the motor/transmission to facilitate a good ground to the electrical system.


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Because you have "no Clue" You best look for an independent garage that works only on european cars and have the car towed there. It shouldn't be hard to figure out what is wrong.
Those that did the install seem incompetent.