It's not just the EC who can enforce the by-laws – any owner, or indeed a tenant, can go to Fair Trading and ask for a Notice to Comply to be issued against these parking space land-grabbers.
The question is, is it a matter of principle or is there a genuine shortage of visitor's parking spaces. The law doesn't make the distinction – it's either a visitor's parking space or it isn't – but your neighbours will if they see you as more of a trouble maker than a problem solver.
Your neighbour seems to make up the rules as she goes along but until a majority of people in your community object to the fact that she and her family are parking their cars at everyone else's expense, then you may be better to live with it … or keep it in your locker until such times as you fall foul of the by-laws yourself.