Re-installing a drain in wet room shower

[Newcastle78]

We've had a leak in our shower, which looks like it is from the drain in the wet room shower and the grouting around it. The water was actually coming up through another tile, so it seems that the waterproofing under the tiles is sound.

I have removed some of the old grout which was around the drain and when this has been removed the top of the drain has come away. It looked like this was secured down previously, and obviously the adhesion has worn away when the water has been getting in. My question is how do i restick this part of the drain (the piece shown on the left in the picture)?

drain 2.JPG (114.47 KiB) Viewed 1664 times

Do i stick it with an adhesive, then grout round the sides, or do I just use grout? Do i need to remove the rest of the existing stuff that was used to fix it in place?

Many thanks!