I think it's more likely that disturbance of the soil has allowed the smell out. Have you seen your red worms yet?
Clay pipes are very often cracked and broken after a hundred years or so, it gets more obvious when there is subsidence or tree roots get into the crack. The damage is usually at the socket where two lengths join together. There are modern sleeved connectors which have some flexibility now.
If your house had no cellar you might not have known about it.
Most of the work will be in digging the holes, so it will depend how deep and how long the hole is. It might take a couple of men a few days if a small job. I doubt they will use a digger or a dumper unless they need to run a new section to the road. Sometimes when you dig up a break, you uncover another break, and when you dig up the second, you find a third. This is not the builders' fault, but can become depressing. you'll want to fix it all rather than come back in a few years. If it starts to look like the whole thing needs to come up, ask if they should reconsider the approach and bring in a bigger machine. The technology and materials are very simple, unless the drains are fit to be relined with plastic.
You had better notify your insurers that you have found a leaking drain, see if they cover it and if they want to inspect or deal with the contractors, but I think you will find they don't cover underground pipework.
I will guess that the next-door house is the same age, built in the same way with the same materials, so presumably their drains are in the same condition, so encourage them to have theirs investigated.
If it was me, I would like to expose the cellar wall near the drain, and see if the mortar has been washed away by the leak - if so, hose out the mud and repair with a pointing gun which is very quick. Surprisingly, it does not do any good to waterproof the face of an old underground wall, because damp still forces its way up from the footings.