“Unfortunately, due to the impact on the financial affordability of the industrial action, we were not able to back-date the increase to the October anniversary,” said Roe.
Plumb said members wanted to receive £17 an hour to reflect the staff’s hard work and that they work in “one of the most expensive cities”.
He said previous “talks have broken down” which has resulted in more action.
The union said its members wanted “a pay increase that reflects extremely difficult demands of the job and the rising cost of living”.
It also previously said it would “escalate” strike action if the dispute was not resolved.
Roe encouraged those striking to “return to work” whilst pay talks are concluded, and said his “door will remain open to the trade union”.