I would carry out a continuity check between the stack and the incoming connected oil service. Perfectly ok to use interconnected metalwork as an earth path provided it is continuous and of sufficienly low resistance. The pipe i assume is bonded as a MBC.
Not really dealt with this type of cooker though. You would also have to judge if the stack was likely to induce a potential from an electrical source. If not having an earthed pipe and insulated stack simultaneously accessible would not really cause a hazard anyway in my opinion.
Ah - sorry - when you said chimney I thought you meant something like this:
Sounds though like you're talking about the flue for an oil-burning appliance.
Does it have a fan in it? Is it an exposed-conductive-part? i.e. is is a Class I electrical appliance? If not, it doesn't need earthing, and as there are no requirements for supplementary equipotential bonding in kitchens it doesn't need that either...
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local