Tax codes

JCS1

The issue is a month 1 tax code basis when they have had no other income this tax year.  if you make contact with HMRC, they should be able to send an updated tax code to your employer.

MalMonroe

This link from the government tells you everything you need to know (and some things you might not need to know as well) -

https://www.gov.uk/tax-codes

  

chrisbur

I assume that you do not have a P45 and signed a new starter checklist advising that this was your only job and that you had been getting jobseekers benefit.  This would allow your new employer to use the emergency tax code (currently 1250L) on a non-cumulative basis.  You should get a P45 eventually from jobseekers which might have a cumulative tax code either way give that to your new employer.  If it has a cumulative code this will then be used by your new employer and your tax due will be recalculated back to the start of the tax year.  Otherwise when HMRC have your full details of taxable earnings they will issue a cumulative tax code to your new employer along with details of your taxable benefit and then your tax will be recalculated back to the start of the tax year.  Once this recalculation has been done if you have overpaid tax it will be returned to you by your new employer.  You will only have to wait until after the end of the tax year if there is any problem which causes a delay in HMRC getting all your details.

EDIT
Just had a look at your old posts.  What have you been receiving this tax year so far, jobseekers, universal credit, furlough, nothing?
Which declaration did you select A B or C on the starter checklist?

Dazed_and_C0nfused

I said hefty because in past employments I've paid huge emergency tax on first pay packages. 

The reason that will have happened is because you weren't on the emergency tax code.

As you have now discovered the emergency tax code is the tax code most people are on.  If you check your Personal Tax Account on gov.uk you may well find HMRC have already sent a cumulative tax code to your new employer, either 1250L or 1256L.

Savvy_Sue

just bear in mind that if your tax is refunded when the 'right' tax code comes through, you need to make allowances when the next tax year starts, because you'll pay tax from the start of the year. 

Setting your budget based on this month's net income could be a sensible move. Putting any extra into a rainy day fund even more so.