Extension built over drainage T-junctions - no access to manholes...

[PTS2]

There are one way valve type things you can put in drains which claim to keep rats out but I have no idea how effective they are in practice. If that stops them getting in then the next stage is to clear them out. A man might not want to crawl into the void but I'm sure a terrier wouldn't have any qualms (I think it's still legal to go ratting with a dog if there is only one of them). If you want to immobilise the gravel in your cavity and not use cement for damp reasons then there are resin products used in paving that might do the trick. All random ideas which may or may not work in practice.

 

[EssexAndy]

There are one way valve type things you can put in drains which claim to keep rats out but I have no idea how effective they are in practice.

This could be a good interim solution (I say interim, this problem has been going on for almost 8 years)
http://drainstore.com/water-rat-and-vermin-non-return-valve-anti-flood-valve-110mm-i605621.html
Knowledge of this product, if it works, could have saved a whole lot of pain...

If that stops them getting in then the next stage is to clear them out. A man might not want to crawl into the void but I'm sure a terrier wouldn't have any qualms (I think it's still legal to go ratting with a dog if there is only one of them).

Provided I've got the walls secured, which I think I have at the moment, I have no problem with filling the drains with poison.
I haven't seen people using Terriers for hunting for years - used to see them at Walthamstow Reservoirs, catching rabbits.

If you want to immobilise the gravel in your cavity and not use cement for damp reasons then there are resin products used in paving that might do the trick. All random ideas which may or may not work in practice.

The current gravel, wire mesh and heavy objects blockade is holding up well at the moment. I'm not keen to make the solution permanent because it isn't fixing the root cause.

 

[theprinceofdarkness]

If you have sealed the area, I can only suggest some rat traps scattered around in the loft will eventually kill them all. The best solution would be to trench under your void to gain access to the old manholes and sort them out, but you will need the traps to kill off your existing lodgers and remove their corpses.
Frank