I've had a look through some (although not all) of my old herb books and I haven't been able to find any references to smorage, the only herb I can think of that sounds remotely like it is smallage (Apium graveolens) which is the wild form of celery, though that doesn't have the healing attributes that you say the herb your granny used has.
Plantain (Plantago major) does though and is about the right shape (round to oval) and I know that it has wonderful healing properties for the skin. It can help promote healing in wounds, soften the skin and even to speed up cell regeneration! I can't find any folk names that are anything like the ones you mention for Plantain though.
In Gabrielle Hatfields book "Encyclopedia of Folk Medicine: Old World and New World Traditions" ISBN: 8182900174 she says about Plantain "... in Scotland and Ireland, there has been a belief that the upper side of the leaf is for healing whereas the lower side draws out poison." so maybe Plantian in Lincolnshire has a different folk name?
It may be an idea to contact your local paper or local Women's Institute as someone local may have the answer. Given the above and the fact that Plantain would be found growing in the places you describe I think this could be your mystery plant. Do let us know if you find out for certain what it is?