Accounting for a pension using cash accounting when self employed

As self employed I need to set up a pension. Can I account for the full amount of any money I invest into a pension pot prior to any tax liability? This would mean that my "profit" would be reduced prior to any tax being applied, much like an expense. Also what sum of money or limit can I put into a pension each year and as I've not done so to date (4 years) is there any carry over from previous years and importantly where should all this information and detail be entered into the self assessment form?

Hi Tom Clark,
Pension contributions do not reduce your profit as they are not an expense.
Please see guidance here:
Tax on your private pension contributions
Thank you. 

Am I correct in that if I contribute to an approved pension, HRMC list which I assume is available but I've not yet found, then an additional contribution by "government" will be applied through the "pension company" for my benefit at source and at a rate that matches my contributions on a 20% basis. So the value of the contributions is netted off for the tax I pay from profits. In other words I get the tax back on all contributions? There is probably a taper for higher earners but I'm a long way from that but it would be good to provide some links so I can read up on that. Thankyou.

Hi Tom Clark,
Please see guidance here:
Tax on your private pension contributions
Thank you.