Auto-enrolment into a pension scheme is mandatory after 3 months service now. You have the option to opt-out, but you would be reducing the total pay packet available to you because the 3% minimum the employer pays is not deducted from your earnings, but additional to them.
One thing that probably won’t be specifically made clear is that you can opt-in from day 1 and not wait 3 months too. If you are moving agency a lot and want to be consistently putting money in a pension, just let them know that you want to be enrolled from day 1.
In terms of getting paid correctly for holidays, unfortunately I have had some bad experiences.
When leaving you should be paid up for accrued holiday and a P45 issued. Sometimes I have left it a while to see what happens and sure enough they will try and get away with issuing the P45 without the holiday pay. You really have to watch them, although not all are that bad.
Sadly I have also had to point out the calculations were incorrect on a few occasions, which is scarcely believable considering that most will have payroll software.
One tip is to be aware of what your holiday year is. Some contracts have it as the calendar year, some the financial year and some your holiday year ends on the anniversary of your start date. Some contracts will have you losing holiday if you don’t take it.