6 years, be out in 4 probably. Hardly worth bothering to catch them.
And they used a 'stun gun' which I presume is a taser. A 'stun gun' is classed as a 'prohibited firearm', mere possession of which can carry a ten year sentence, let alone using it to commit a robbery. Somewhat lenient sentencing IMO, perhaps things are different in London.
"Stun guns are prohibited weapons and it is an offence to possess them. A stun gun is a weapon that discharges a large voltage of electricity into its target, rendering them incapable of movement for a short time and are very dangerous. It is illegal to import or mail them into this country and the penalties will apply.
If you are found in possession of a stun gun, you could face up to ten years imprisonment and/or a fine. In order to possess, purchase or acquire, or manufacture, sell or transfer such weapons you must have authority from the Secretary of State."
https://www.westyorkshire.police.uk/.../question/Q509