The electricians got rid of an old socket no longer needed, and secured with a wago type connector. Can i simply plaster over this with bonding coat prior to skimming?
No.
The plaster will contain moisture which will seep into the connections and if it does not go bang then your RCD if you have one will likely trip.
use a blanking plate.
You would be better off with a blanking plate.
If you cover them (you will not get the wago's in a wagobox within that space) then what happens if something goes wrong?
you will have to break through the plaster.
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