Russia still relies heavily on trade with the rest of the world, particularly oil. Sure it would hurt us to put sanctions on Russia but it would hurt them a hell of a lot more. We can buy oil from elsewhere, they will struggle to sell it elsewhere. Putin was doing the old bully tactic of picking a fight where he thought there would be no retaliation. Like all bullies, if they are faced with someone who is prepared to front up, they often back down. It's taken the world too long to realise but the more we let Russia act without consequence, the more they will do it. Putin and Russians want to be the hard men of Europe again and have got it into their heads that they are because nobody has done anything about it.
Nope the expelling of diplomats must be followed up with action to let Putin know everyone has had enough of his pseudo-KGB bullshit. Let him act the tough man at home and the longer he acts that way, the greater the pressure we should apply.