Jock1e

From the rotten egg smell description, I'd say it's something in the exhaust system - probably sulphur from fuel burning up catalytic converter.

No easy way to say what the fix is, but you can try fuelling at a different place. The diesel in an alternative fuelling station may be from another refinery and have a lower sulphur content. It may also be that the car does it anyway, but your brother in law runs the car a lot more so sulphur deposits don't get a chance to build up - if you only really do short run journeys (say, a couple of miles) then this is likely the cause. Try taking the car out for a long run and see if it smells the following morning on start-up.

The alternative is that the car is running very rich and leaving fuel deposits on the cat - that could have any number of causes.

Opps forgot to say try Esso fuel we used that if we had a problem as a known good fuel,how do i know let's just say i used to work in a large motor manufacturers technical centre.

Start the car and get out and smell the exhaust fumes, I have noticed some cars produce a very sulphurous smell when they have just started up.

Quickbeam

"The rotten eggs smell is hydrogen sulfide caused by an over charging battery"

Plausible, but highly improbable.

Given that the car has only been making short runs, the battery is far more likely to be under charged than overcharging.