When my white Special is being driven with the engine under stress, eg going hard up a hill, there's a noticeable smell that is kind of oily and sweet and fumy - but not really exhaust fumes. Also, there's a kind of rattle that sounds like a sort of slurping noise.

The smell goes when the engine is under less stress, and so does the slurpy noise.

Any 2CV doctors care to give a diagnosis?

Ooh 'eck!

Are there any threads on here that will show me how to repair my engine?

Unfortunately, it will be eroded cylinder head seating, allowing combustion gasses to enter the heating system.
Most likely cause is prolonged detonation, caused by incorrect ignition timing, spark plugs with too hot heat range or an air leak in the induction system.

If you're lucky, retorquing the heads may restore the seal or reduce the leakage, but since carbon monoxide is part of the mix entering the cabin, a full strip down and repair would be preferable.

The noise is probably pinging/pinking, but that's usually described as light metallic crackling or rattling, like nails being shaken in a bucket.

ken

p.s. Now Uni Challenge is finished; if you're going to keep using the car, your immediate priority should be to sort out whatever has caused this problem, starting with the ignition timing. Have you got a strobe?

well lets hope the "bargain123" hasn't cost you a cylinder head.
unfortunately its as its going like a train at 75mph when all the damage has been done DG has a habit of over advancing the timing and so its been pinking away merrily, this is why it is so important to check the advance on these engines - and you can only do that with a strobe and some white marking paint. The peg in the hole is now relegated to initial bench set up and finding a datum on the flywheel.

if it had been set by strobe the worst that would have happened was a slightly lumpy idle

- bet you've been enjoying the "efficient heater" though?