Do you know the make of the waste?

Most, if not all, modern shower wastes are designed to be worked on from above, that way you can clean out any trapped hair and debris. So you shouldn't need to dig up the base.

Some traps seal better than others, I have a Matki high flow trap and it is particularly poor in this respect; perhaps you have the same or a similar trap.

With these the cover should easily fit off revealing a shallow well. This has a tight fitting cup in it which holds water and forms the seal. This can be lifted out to clean it because it takes very little in the way of hair in its slots to slow the water flow down dramatically.

This cup should be pushed firmly down into the trap and the cover replaced. The cover should seat, turn it slowly until you hear it click as it drops down a couple of millimetres. Then run the shower to fill the trap and create the seal.

On mine, if the cover doesn't drop down by those couple of millimetres, I have exactly the same problems as you, with air and smells whistling out of the trap.

I hope this helps.

Thanks for the advice, will try and post a picture of the waste tomorrow, I am assuming it's not down to a blockage as the smells started within a week of us starting to use the house. There is an outside vent pipe as well but not sure how much of the total system is served by it. will try and find out what make the waste is as well
Thanks again

If you look at the top of the forum there are instructions on how to post pictures. It must be relatively easy because even I can do it

What the hecks that, it looks like a bits missing in the center.

Looks a bit like a TSG1, McAlpine trap,(ones I like to fit in a tiled shower)  without the top chrome grid, but some 'erbert has whipped away the removable centre which forms the trap....

wallies!!! (being polite here...)

and a link to a TSG1 if needed...

http://www.mcalpineplumbing.com/tsg1-t150ss.html

and without being nasty, just honest, if they were professionally installed, and recently, I'd have the builder back, and the tiler, and get them to install the tiles correctly to start with.... I would sack either myself or my tiler if I left a job looking like that!!

Two of the traps are identical I.e. As in the picture
Neither has a centre section, and the removable grille is square ( very sharp cornered)

3rd one is a different trap, has a black plastic disk that has holes in it under the outer cover
( guess that is a hair trap)
I have asked the builder to let me know exactly which traps were used, which hopefully will give you a better idea if something is missing
Many thanks

I wonder if someone has removed the centre part in order to try and improve the flow rate? That centre part is essential as it performs the U bend function.