HEEELLLLPPPP!!!!
I have a 3.0 Elite Estate 1999, 150k on the clock and for my sins unleaded .
the putty smell occurs when I accelerate, even slowly, so on the way home from work thought I'd see if it happens when you accelerate hard, imagine my surprise, it did but this time at the bottom of the windscreen (drivers side) condensation, vapour appeared. the smell of putty was stronger.
there are no leaks known on the engine, checked radiator, expansion tank. Even looked for putty 
But nothing please can anyone help or had these symptoms before
Regards H

Looks like you may have heater matrix staring to leak

Thanks for the replies , yes it is a sweet smell and had a good look and the carpets are damp on the drivers side.

there doesn't appear to be any lose of coolant. the car, cabin, does feel warmer than it should be.

is it a big (expensive) job to sort out
Regards
H

Another thing to bear in mind is that the control flaps in the A/C system are actuated by vacuum from the inlet manifold. There is a vacuum reservoir that should keep the system working when you floor the throttle and the inlet vacuum is lost.

On my pre-facelift the non-return valve wasn't working and you could hear the control flaps shutting on full throttle and creaking back to their correct positions when part throttle was resumed.

I can't imagine a faulty non-return valve causing the smell but it could be the cause of the screen steaming up momentarily.