A copy of your boiler or appliance manual should be retained at the property. You may often find it has been left in a kitchen drawer or cupboard. Occasionally this is accidentally removed. The best source for up to date manuals is the manufacturer. Most manufacturers provide copies through their website. You may find the following websites useful.
For boiler manuals https://www.freeboilermanuals.com/
For appliance manuals for Electrolux, AEG and Zanussi https://www.serviceforce.co.uk/appliance-troubleshooting/appliance-manuals/
For user manuals for many household appliances: https://www.espares.co.uk/usermanuals