Had this same thing happen in my old house, luckily enough we had an insurance policy where this was covered in it.
The drain people came round and tried to unblock and couldnt, the poor chap was there till 2 in the morning and it still wouldnt unblock.
Turned out is was a scummy bugger who lived 2 doors away, apparantly his drains had been backed up for weeks but didnt think to do anything about it.
Luckily enough the council came and basically blasted the whole system, it eventually shifted.
But man...i feel your pain, we lived with the stench for weeks wondering where it was coming from.