You carefully prise off the circular cover, it doesn't unscrew - there may be a small notch at the bottom of the ring, that a screwdriver may be inserted into, to help lever it off.
How do you know it’s not screw on variety ?
Not too me , the screw on has a sharper angle on the edges.
www.axminstertools.com
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.
Are you a trade or supplier? You can create your listing free at DIYnot Local
