Hopefully someones been in the same boat as me in the past!
I’m due to start my first stint of agency work in a couple of weeks alongside my (currently) full time job. Since I have yet to recieve a P45, this would to my knowlage automatically put me on the emergency tax code at 40% 
Would this entitle me to a tax refund the following April after recieving my P45? And if so, how do I go about it, is it done automatically or is it an accountant job?
Cheers 
You may need to register for self assessment to get the easiest rebate on any tax overpaid. However ringing the tax office once you’ve received a P60 from both employers may qualify you for a rebate, as they’ll be able to see how much you’ve earned in any tax year, and correct the tax paid.
I thought tax on a second job was charged at 20% of all earnings, as the presumption is your personal allowance is used up on your first job. But I stand to be corrected on that.
The agency should have asked you to complete a P46, which you can download for yourself from the hmrc site.
The bottom left has a section which states “I have another job” complete this and sign.
You will pay basic rate tax at 20%, however if you anticipate getting a large part of your income from the agency as opposed to your existing job you can ring the tax office to ask them to allocate your tax allowances across both.
If you are due any, you will automatically get any rebates at the end of the tax year. (I did)
Sorry, just checked, P46 replaced by a Starter Checklist as of April, even though I have completed one this month!
New form Redirect Notice
Hope that works as link on hmrc site didn’t. That link worked for me…
Emergency tax code simply gives you one twelfth of the usual annual allowance with your new employer until HMRC get themselves sorted out. For most people that means you’ll get your tax-free allowance plus a chunk taxed at 20%. Only if you’re highly paid will you get a further chunk taxed at 40%.
PAYE is electronic these days, submitted through a process called Real Time Information (RTI). That means as soon as you get paid through PAYE, HMRC will find out your pay amount, make some assumptions, and issue a revised tax code to your new employer. It should all happen within a month or two so you’ll be on a better tax code soon enough.
However, if your employment is complicated (multiple employments) or your employer ■■■■■ up - as did mine recently - you’ll end up paying the wrong amount of tax for the year. It doesn’t need an accountant to sort it out if you’re okay filling in forms. Ask HMRC to send you a self-assessment form and follow it through.