A sinking fund is not fancy
If car insurance is GBP 600 once a year, the real monthly cost is GBP 50. If Christmas normally costs GBP 720, the real monthly cost is GBP 60. If the school uniform hit is GBP 300 each summer, the monthly cost is GBP 25. A sinking fund simply tells the truth earlier.
MoneyHelper's budgeting guidance notes that costs can be entered annually where they vary or do not happen monthly. That is the same mental model. You turn irregular costs into monthly set-asides so the bank balance is not lying to you.
The normal household list
Car and transport
MOT, servicing, tyres, insurance excess, repairs, train season ticket, parking permits.
Home
Boiler service, appliance replacement, repairs, furniture, insurance excess.
Family
Christmas, birthdays, school uniform, trips, childcare gaps, prescriptions.
Admin
Annual insurance, professional fees, memberships, tax bills, passport renewals.
The simple calculation
Monthly sinking fund = expected cost / months until needed. If a GBP 480 car repair target is due in eight months, set aside GBP 60 a month. If you can only set aside GBP 30, the gap is useful information: reduce the target, extend the date, cut something else, or prepare another route.