Don’t allocate your tax code to any agency NOT doing PAYE.
Example:
Agency One (Designated by you as main employer) This one has your tax code, BUT as a result you’ll be able to deduct less expenses, since you’re paying less tax in the first place. Make sure your main firm is ideally the local one giving you local work. This will likely be the main work you’ll get, but be careful not to spend more than 40% of your working time with the same client. Ask the agency to give you work at 3+ clients over the year ideally. An example 3 clients might be TNT, Tescos, & Stobarts.
Agency Two (Also PAYE, but work is very bitty) Better hourly rate, but you have to commute for instance. Pay the emergency tax, and have a bigger taxed amount you can deduct milage from.
This might be the work you get during the school holidays. Example clients here might be MrCT, Langdons, or Samworths. 
Agency Three (Inisists you go ‘umbrella’, meaning it’s NOT paye even if they lie and say it IS!
) Use this firm only for top-dollar work eg. £13ph plus stuff, or £18ph if you have ADR. Effectively, this will probably price you out of the market, but who cares? - They lose more than you do if you don’t get any work from them, as you’re effectively paying per shift to work there. Top dollar makes it worth-while, but you’re only likely to find yourself getting last-minute bank holiday cover or being phoned up sunday afternoon to cover a sunday night sickie, and the like as I’ve found. Examples here might be Weekends & Bank Holiday sick cover at Morrisons, Iceland, or Co-op. 
As long as you don’t spend more than 40% of your time working out of the same depot, you can claim milage. If you get all your work from only one or two agencies on the other hand, then it is highly likely that you’ll be sent back so often to the same old clients, that you ARE there more than 40% of your working year, and will fall foul of HMRC - so why do it? 
Have 3+ agencies under your belt at all times, even if you get hardly any work from one of them! 
The over-tax you paid throughout the year means you get a nice tax rebate set up around about now. You’ll need to contact them with your gross taxable to date amounts from any agencies you have worked for over the tax year just finished, and they should do the rest. HMRC are a lot easier to deal with by phone these days than they used to be.
If you get tax credits, you’ll need to also set up a final lump payment (them to you!) when you have your final aggregate total “taxable gross to date” for the year too. Tax credits are done on a different number at HMRC, but is well worth the effort.
The lump sum comes from the fact that you declared your estimated earnings for the year earlier on, hopefully over-estimating your total. Your final figures will be reduced by the expenses credits, and your tax code to give you the “real” total which should hopefully be well under your estimate (because you didn’t know what expenses in advance to knock off right?) 
Eg. Your earnings to date might be £17,000 less the expenses & tax code which leaves you £9,300 taxable for the year.
You declared however £10,500 taxable for the year, so the Tax credit department send you a lump sum of around £350, and you’ll get a tax rebate of whatever you paid at the OTHER non-main agencies throughout the year.
Thus, if you didn’t do much of that stuff, you’ll not get much of a rebate, but you should still get something. You might get as little as £250, or as much as £1200 with the figures shown above for example.
All worth doing however, as anything HMRC send you is in itself tax-free, so it is effectively “free money” which has got to be the most popular kind for anyone I would have thought!
