Not sure if i'm posting in the correct forum here but as the title suggests, my girlfriend's first payslip (new job) has a negative figure under the Deductions column so she's actually been credit with tax! The tax code is 548L. Could this be a payroll error??

So it's an Addition rather than a Deduction? - This happens when tax has been paid before so my guess is that she has been working before and was taxed at that job on the assumption that she would work there for the full tax year (it's the PAYE system), then she took some time off before starting this job.

As an example, suppose someone worked only for the first and last months of a tax year on a monthly salary that, if it was for 12 months, would come with £100 tax deducted each month. The payslip for the first month would have £100 tax deducted while the payslip for the last month would have £100 tax credited as the total earnings for the year would be within the personal tax allowance so the liability (and payments) for the year would both be £0.