The legislation is the Local Government Act 1972 s85 (1) which states that
"Subject to subsections (2) and (3) below, if a member of a local authority fails throughout a period of six consecutive months from the date of his last attendance to attend any meeting of the authority, he shall, unless the failure was due to some reason approved by the authority before the expiry of that period, cease to be a member of the authority."
The emphasis is on the fact that if the reason for absence is approved then the clock resets. Therefore some councillors could play this card, turn up once in 6 months and still retain their seat. In our council, a resolution was agreed that no apologies or reasons for absence would be accepted, just noted, which means that the clock cannot be reset.