Swivel plug stuck with grit

Hi,
New here. I'm a freelance Web Designer so not a Plumber so please excuse my naivety - well that's that covered, lol.
I have a couple of issues. We had a new bathroom refurbishment in 2015 but have had a few issues and the original builder just wasn't interested in assisting or advising. We had way too many blocked toilet scenarios and eventually got someone in to assist. Someone had a look and thought our pipes were collapsing so advised to get the insurance involved. Well Insurance company sent out their person who had a look with a camera, then the report came in. The Insurance said it was "bad workmanship" and something they did not cover. Apparently indoor piping used outdoors was their assessment. It looked as though a reduced had been used to connect new pan to cast iron works.

A friend got involved and sent someone to find the problem piping; as we had no real idea where the fault was located the whole floor and shower tray have been bug up. They found the piping; a white insert had been used to connect the two 4 inch pipes, effectively causing a bottle-neck and reducing aperture to 2 inches. This reducer was also embedded in a layer of concrete. In the process of digging up the floor some grit got into one of our bathroom sinks and seems to have jammed the swivel plug. It is totally jammed. I wondered whether it is possible to remove the mechanism by loosening the hex ring beneath the sink and perhaps lift out the plug? Do you get a appropriate spanner in B&Q?

Whom ever has used a multi kwick pan connector to couple two soil drains together in a horizantal connection...all wrong wrong. more later ..centralheatking

you will need to tell us more about the 'hex ring plug'. for more help
piccy ...centralheatking

The nut shouldn`t be that tight so either a pair of grips or a spanner to remove the nut is ok. There are two different basin wastes depending if the basin has an overflow or not, Simply replace the waste @ around £11,
Posted @ 13.48.

Indeed the wrong connection has been used there are purpose made rubber adaptors made specifically made to adapt from one material to the other and avoid any unnecessary reduction or step in the joint, the waste will need replacing which is a simple job for a plumber. Kop

Thanks for the responses. A friend sorted out the collar and connection.
I just have the issue with the stuck swivel plug now.

Now this may work it might not

Put some water in the basin around an inch or two

Get a piece of wooden dowl and on one end eg the swivel give it a tap eg put the wooden dowl on the waste swivel and tap the dowl with a hammer not too hard and don’t drop the hammer

Once it’s open clean all around the internal parts where it seats up against

Either that or change the waste