It depends how mech & tech minded you are.
If your happy with the location of everything and removal of seats/trim then yes by all means check the wiring in the areas highlighted above.
If not then just take it to a dealer. you may get charged a diagnosis fee, prob around 1/2 hour labour, this is what my dealer would usually charge but i have VCDS. That would just be for them to tell you what the problem is without doing anything about it.
But as i said the dealer will be able to follow the guided fault finding they have and from that they would be able to give you an idea of cost.
Worst case scenario i would say £600. Thats if you needed all new wiring, Igniters and labour on top. I would be surprised if it was both wiring and hardware.