THE WHIPS - PAIRING
An MP who wishes to be absent from voting on a two line 
whip can get round the problem by using the informal 
system known as 'pairing'. This allows a member from 
the Government side to pair with a member from the 
Opposition side (not the minor parties) so that their votes 
cancel each other out, and neither need be present. The
system depends on the co-operation og the two main parties
and does not always operate. It can only work well when the two
main parties have a fairly similar number of MPs.

These pairings are recorded in advance by the Whips, who are normally happy to grant leave of absence, except on occasions of political importance i.e. a three line whip.