The cameraman probably laughed or sneezed as Mr. Humphries id "rip her tight elastic", because the camera shakes.
At 11:42, over the ladies' counter.
Mr Rumbold quickly steps in to correct Mr Grace when the latter gives the wrong amount concerning the £200 cheque. This was presumably to avoid having to retake (at least part of) the whole scene.
When Mr Lucas is commenting on his payslip (paycheck), he says he should get £19 per week before deductions and after he got £11.47.
The deductions he lists are: Tax £6.12, National Health £1.21, Grace Brothers Social Club 10p, Grace Brothers retirement home 20p. These deductions added to his net wage equal £19.10 and not £19 exactly.