Were UK Uncut right to do this? Yes: decidedly so. They caused no harm; this was peaceful protest. They showed Hartnett mixing with people who wished to call UK Uncut "trespassing scum" and who wanted "to set the dogs on them." Remind you of Andrew Mitchell anyone? In the process it is clear by his presence that Hartnett showed in whose interests he thought he was running HMRC. These were the people he wined and dined regularly.
I should add that way back he once lunched me (at the Cinnamon Club, of course). And he also invited me to speak at HM Treasury. That was in the days when Dave said he was going to make sure tax avoidance did not pay and he thought I might be of use to him. But Dave changed his mind about making tax avoidance pay and we've all paid the price for that.
UK Uncut have rightly drawn attention to this.
We don't need Hartnett's like again.