To anybody looking for a decent converter plug for wall warts, I can fully recommend the products made by this company
https://www.powerconnections.co.uk/shop/converters/euro-to-uk/pcp/
Searching for "powerconnections pcp" will give you suppliers for the unit. The lid is held in place with screws, and two rubber pads grip the transformer to stop it falling out.
If you only need something less permanent then this will do nicely
https://www.powerconnections.co.uk/shop/converters/euro-to-uk/bca/
https://www.powerconnections.co.uk/shop/converters/euro-to-uk/pcp/
Searching for "powerconnections pcp" will give you suppliers for the unit. The lid is held in place with screws, and two rubber pads grip the transformer to stop it falling out.
If you only need something less permanent then this will do nicely
https://www.powerconnections.co.uk/shop/converters/euro-to-uk/bca/