Hi Lyonaria,
It's been a while since I've had full time pay slips etc so bear with me..
Take a good look at your payslip and the payments and deductions and make a note of them. Look up online what each aspect means, and I mean that in case you have deductions for a pension for instance. Also look at your contract of employment as I don't believe they should be able to deduct pay if you've missed days due to being sick. If the contract does indeed state they can, then that's probably why you've got different amounts. Similarly, if you've worked extra hours, the contract will say if they pay for that which could be a reason as to why.
Do check the Tax Code as well, keep in mind HR and Finance departments in many companies are just a person sifting through everything and mistakes often happen. Another good thing to do but 'seldom' done by the masses in the UK, is to buy a personal taxation software solution and complete it for your own situation. At the end of the year, when you submit it, it may well be that due to mistakes by the employer in your pay, you could get a tax refund.
Overall though, the situation of pay being all over the place is extremely common over every employment sector and business size. As an example, there's a company that's the largest employer within that sector in Europe - the UK has around 23 locations and recently, almost 'everyone' has had their pay completely and totally messed up for 2-3 months now. I'm talking BIG errors, like overtime payments for a month not paid meaning some employees are missing £1000+ from their pay packets!
Hopefully, you'll be ok with your employer soon, but do keep in mind the above aspects so you can hopefully get the money you are owed/not pay too much tax etc etc.
Cheers, DtM! West London & Slough UK!