In most cases the catch is accessed by removing the hob from the top, what model of range do you have.
This isnt a consumer serviceable part, which may be why your struggling
Since this is a gas appliance I would recommend you call out a gas safe engineer, who can fit the part then retest the appliance, as it will need recommissioned.
There will be a gas council number on the rating plate which the engineer can use to lookup the instructions for fitting the catch.
If you would like me to recommend a engineer please provide your postcode.
Good afternoon, my name is ***** ***** just wanted to follow up to see if you required any further assistance as I havent heard back from you recently?