You need to estimate an average annual amount for those things and put money aside in advance. It doesn't matter if it's exactly right, as once you get into the swing of things, you can build up a float to cover these things.
You need to stop thinking of all your income being available to spend this month. School shoes, car repairs, school trips are not unexpected costs. As a homeowner with DC and a car, these costs will arise, sooner or later and can add up to a decent amount.
For uniforms and school shoes, you've just spent £300, so that's about £28 pm, more if you'll spend more on uniform within the next year.
You probably need to put by at least £50 pm for car MOT, servicing, tyres and other repairs.
For insurances, look at the cost of last year's premium, and divide by 12 to get the monthly amount.
A good start would be to put £2/300 pm into a savings account that you only draw from if you've spent on something 'extra', but see if you can manage not to withdraw it all, so that a surplus builds up. You could try having a frugal few months, or selling unwanted possessions, switch your bank account and utilities for switching bonuses, etc etc to give your 'irregulars' pot a good start.