Slow bath flow and back feed from basin to bath

Hi

For some years we have been putting up with this problem.
The basin runs away quickly but unless the bath plug is pressed in hard water (sometimes smelly) will force its way into the bath!
There is also much gurgling after the basin has drained away.

I thought it was blocked and so today ran a drain auger through but little of note was found.

However I am very surprised at the baths trap etc.


It does not look right! the water will flow directly into the overflow pipe first. Plus if you look at the levels its no wonder the bath is very slow to drain and that the trap water level is always just under the plug hole!

Questions is what can I do to sort this out, 1970's vintage plumbing here?

Thanks

Mike

Just to add to this.

The run from the bath past the basin towards the toilet and outlet is not very steep.



And here is whats behind the basin (must get the hover out )

1970`s tiles too ..where`s it go after thru the wall to the right of the basin ...if this could be lowered , you could re do it all and use Hep VO waste valves instead of traps ..........not Manor Park again ?

Hi it runs to the main down pipe (soil vent) junction.

Is there such a thing as a one way valve that would stop water from the basin going into the bath waste pipe?

Mike