I've been self-employed as a sole trader for over a year, but now I want to form a private limited company and trade via that (I'm in IT, and some organisations here won't contract an individual directly due to IR35). I understand that I would effectively be working for the company, so do I create a payroll and give myself a salary? Are there restrictions on how that works? Is it still OK to take different/no payment during "lean" months?
You are required to inform HMRC that you will be employing people, even if it's only yourself. The main thing to be aware of is your obligation to withhold income tax and National Insurance under the PAYE system, and to pay employer's National Insurance contributions.
As for paying yourself less/nothing, that should be fine. As a company director you are excluded from the National Minimum Wage regulations unless you also have a contract of employment with the company.
One seemingly common approach is to pay yourself a small annual salary that takes advantage of your personal tax allowance (£6,475) and then take the rest in dividends. You may want to consult an accountant to make sure you come up with the most tax-efficient arrangement.
You should probably read through the pages on the HMRC website. Don't pay too much attention to me as I am a novice at this too.