Shower base; regrout or reconcrete?

Question

Hello,

a year ago we built an ensuite.

The stumps, joists, subfloor and plumbing are all new.

The shower uses a shower tray from Highgrove Bathrooms.

We had the tiles installed professionally, but a year on, we noticed the grout around the shower drain breaking away.

Over time it got worse and we began to noticed movement on the edge of the tiles adjacent to the drain. When we applied pressure to them, water would squish out of the edges.

At this point I contacted the tiler and he said that either;

* 
the foundation of the shower was moving and that would require a builder

* 
the plumbing is leaking and that would require a plumber

The house is on stumps but I am unable to get underneath to visually confirm water leakage or structural movement beneath the shower base.

None of the other tiles or plasterwork, in the bathroom, have any cracking or signs of water damage.

We haven't used the shower in a couple weeks, to allow it to dry out, and today I pulled up the drain grate and found some of the tile cement to be loose. There is no sign of moisture.

My next step is to call in a new tiler but I just wanted to see if any of you have any experience or suggestions?

Thanks