Bribie, I've used Nottingham for probably a good 25 + brews, it has very occasionally with brew temps below 17 degrees smelt more yeasty as far as the smell it's putting out, perhaps that could be interpreted by someone else as sulphur. As I've mentioned already, I've never had an infection with Notts, it's all been clear sailing - some are a fan of it, others have espressed a dislike because of a perceived 'dusty' characteristic to the taste, whereas I don't tend to get that (which is not to say that it's not there, just that I don't get it.) I have usually found it to be a monster of a yeast, especially at 20 degrees where I routinely get massive krausen, huge amounts of CO2 with a very vigorous ferment finished in 2-3 days. I've had one knock over a 1.049 beer to 1.005 or so in 34 hours. Is it possible that because of the vigour of the ferment and large amounts of CO2 production that you are noticing a stronger smell? Otherwise I would possibly put it down to the additions, which I couldn't comment on as I don't make those kind of water adjustments (although Warra's comments seem to make sense to me). If usually your resulting fermented beer is sweeter in comparison, it may be that Nottingham had eaten a few more of the sugars than your usual yeasts, and as a result you have a much drier beer (higher attenuation) that you're not expecting or used to? It does tend to be a very hungry yeast, I have had one brew finish at 1.003 with it (mashed bang on 62 degrees for 70 minutes), and it was thin as p*ss because the yeast had eaten everything.
Overall, it's a good reliable yeast - I think that it has in it's favour that it usually starts too fast & ferments too vigorously for any infection to be able to get a chance, so I recommend it to new brewers routinely - it removes the 'complication' that yeast can cause for brand spanking new brewers.
