You need a Hepvo valve.

No. I fitted one under the shower floor with the intention of fitting a grey water tank ... did not get that far. The Hepvo valve is close enough to horizontal mounted close to the shower floor and then a fairly shallow fall to the outlet. Works perfectly and sink or vanity water has never come in.

I would have thought that the issue is that the water is not draining away from the junction of the sink pipe and the shower pipe quickly enough. The high/sudden flow from the sink is not draining away quickly enough, so a backup occurs and the water flows back up the shower pipe.
The backup could be caused by a blockage in the pipe from the junction to the outlet, a rise (or no fall) in the outlet pipe, or the outlet pipe being of too small a capacity for the application (just not able to handle the sudden volume of water).

To me, the solution is to investigate the outlet pipe from the junction to the discharge point. Improve the fall, reduce/eliminate any rising.
Replacing this section of pipe with one of a larger diameter would be high on my "to-do" list.

I have seperate drain hoses all the way from the sink and the shower to the bottom of the grey tank. No smells and no valves are required.
Increasing drain pipe size is not necessarily an improvement. Smaller can be better.
The advantage of a smaller drain is that a syphon can develop which will increase the flow rate significantly.
Cheers,
Peter

I have ordered the Hepvo valve, so we shall see if it fixes the problem, thanks