I was finding my PITT artists pen was giving much too thick a line for this type of sketching, even using the fine tip, so this time I used a Rotring technical drawing pen with a 0.25 nib. I think it's going to be much more useful and have ordered a 0.13 nib for an even finer line. The ink flow can be a little tricky until it gets going properly but you can't beat these types of pen for really fine drawing.
It's such a beautiful day outside - deep frost and bright blue sky - so I'm going to take Misty (Irish Wolfhound) out for another walk. Don't think I'll try any plein air drawing though - fingers would be frozen stiff within seconds!