First Step is to raise your concerns with the nursery manager.
To help you prepare for that meeting, here is a link to the National Standards for Full Day Care (applies in England) - see page 12, section 2.7 for Staffing Ratios.
2.9 is also of possible interest
"there is a minimum of 2 adults on duty"
I had a discussion with a senior team manager at Ofsted (with whom I worked a few months prior to the incident), regarding Ratio's in nurseries - particularly baby units. My view was that 2.9 applied per room. The Ofsted Team Manager did not agree with me, said that some nurseries would apply ratio's throughout the nursery, not on a per room basis - and alas not a lot could be done about that. This was over 3 years ago now, so things may have changed, but I doubt it. I feel the National Standards need to be rewritten to make it clearer, if Ratios are per room (as clearly a person can not see through walls) or the 'premises' as a whole.
Also see page 26 of the PDF, for Annex A - Babies and Children Age of 2 - for things that apply specifically to the care of babies in a nursery environment.
OP
I was told on the first occasion it was because other staff were doing after school pick ups.
That would be a breach of Standard 2, as the staff were not on the premises.
Today it was apparently because other staff were on their lunch break.
I consider this to be a breach of Standard 2, as:
Standard 2.12 says
"The adult:child ratios relate to staff time available to work directly with children. Additional staff and management resources may be required to undertake management tasks, prepare meals, for domestic tasks and to
maintain premises and equipment."
Alas 2.12 does not specifically mention Staff Lunch - but to me a staff member on lunch is not "available to work directly with children".
Discuss with the nursery manager. If you are not happy that the situation will be resolved, remove your child from the nursery - and lodge a formal complaint to Ofsted, so that Ofsted can help the nursery comply with the National Standards.