Once I know the part I can guide you through the process, however to be honest you may find the manual reference above suitable, as it shows you the order items fit together.
I have had a look around and there appears to be generic electrodes you can pick up for under £10 such as the one here, however reading the parts manual it would suggest they each have a different height to allow for the different burner sizes, so I would give the manufacturer a call directly in terms of source a replacement part.
Once you have the part you will need a set of allen or hex bits, and discount the gas and electrical supply from the appliance.
Next you need to remove the trivets, wok ring, and back plate.
At the rear you need to remove the screw either side holding on the hob top (see item 13 on page 8) and the screw on top in front of the middle burner (item 2 on page 8) - note there are often other screws around the burners but I cant see any marked on the parts diagram.
The hob top should then lift up once all screws are removed, this should expose the burners and electrodes.
Disconnect the electrode spade terminal from the spark generator by pulling it with your fingers.
You then need to remove the spring clip (eg item 18) with pliers, and may find it useful to spray WD40 or some other penetrating liquid around the electrode to help loosen it.
The electrode should then come free.
Its then a case of inserting the new electrode replacing the spring clip, attach the spade terminal to the spark generator and replacing the hob top and screws.
bruners and resting key