Water in WC sucked out and 'glugs'

[davidmash]

I have an outside WC connected to a septic tank somewhere in the garden. I don't know where as everything is either terraced over or overgrown. It has worked fine for years but recently when left alone the water in the WC is sucked back (nasty smells, as would be expected) and if used and flushed fills up the pan and then drains away slowly.

Obviously some sort of blockage but how do I start to trace its location and clear it?

Any advice much appreciated.

 

[giblets]

Perhaps the septic tank needs emptying?

 

[sanj.varah]

not familiar with sceptic tanks but i reckon your vent must be blocked.

 

[corgiman]

as giblets says the tank is full and a vacuum is being form when you flush, and when it clears it sucks the trap (ewwww)

get the gulper boyos in