OK, after much scouring of the city I'm answering my own question for the benefit of others who might be looking for a trusty steed that can cope with hills.
Mr Nguyen Quang Trung is your man. Apparently he trains Hanoi's road racing team (there is one?!?) & is a bike mechanic and enthusiast - the real deal. Contact details: 51 Doi Can, ph 0912 410 562 email trungbike@yahoo.com.vn. He doesn't speak any English but his daughter Trang (ph 090 3232 888) does. His place on Doi Can doesn't seem to have a bike shop sign at all and the ground floor is actually a tailor shop, so don't be put off and think you're at the wrong address. The bikes are 3 or 4 stories up. (He normally supplies the tour companies here rather than dealing directly with tourists).
Good luck and happy pedling!