The general rule for pension contributions is that they are allowed for tax when paid. So in theory that means that the balance on the Pensions Liability account needs to be split between Employee contributions (a simple liability, no issue) and Employer contributions (which then represents unpaid pension contributions).
When I prepare accounts for companies, my instinct is that these unpaid contributions at the year end in respect of auto-enrolment schemes should be allowable for Corporation Tax, though, because they are a statutory obligation of the company, not a discretionary payment which could be cancelled.
I looked through the HMRC Manuals, and could only find reference to the 'tax relief when paid' option. I just wondered if anyone had come across this and formed a different view, and what their supporting argument was. In most cases this will not be a material figure, but its an irritation to have to adjust the Comps each year for b/fwd and c/fwd creditors.