The responsibility for funding the scheme deficit lies with any employer who employed a member of the DB plan at any time between 2nd September 2005 and 31st December 2011 when the DB plan closed for future accrual.
The following document, issued to all scheme employers in March 2011 explains why this is the position under pensions law.
Update on Scheme Deficit Funding and Future Employers Deficit Recovery Contributions can be found here Just Group Buy Out Employer Communications.