David Cantrell wrote:

As I understand it, No2EU are abstentionists so if elected they won't
take
their seats in the European Parliament (a bit like the Sinn Fein attitude
to
the Westminster Parliament).

The difference is that Sinn Fein aren't allowed to take their seats at
Westminster, whereas No2EU would be allowed to.

No Sinn Fein are allowed. They refuse to do so because they believe taking
their seats would be recognising the legitimacy of the UK to govern Northern
Ireland. (They used to boycott both the Dail and Stormont as well.) If Sinn
Fein wanted to take their seats nothing could stop them. They do make use of
Commons offices, claim expenses and have sometimes been around Parliament at
a higher rate than various non-abstentionist MPs.