As a professional consultant can I be sued if I give free advice?

Technically you could as a recent legal ruling (Burgess and another v Lejonvarn [2016] EWHC 40 (TCC)) has found that a professional consultant owes a duty of care in Tort for professional services performed gratuitously.
The consultant in question had given various free services relating to a friends’ landscape gardening project. It was a significant project approached in a professional way, with services provided over a relatively long period and involving considerable commitment on both sides. The project did not go smoothly, the relationship broke down and the former friends claimed for the cost of remedial works.
This highlights a risk common to any professional who offers informal advice, however, it is important to remember that this was not a piece of brief ad hoc advice given in an informal way.