It's very old...
Manufacturer:Thorn EMI Heating
Model Marathon 56/76c
Fuel Type Gas
Boiler Type Conventional
Method Non-Condensing
Flue Type Open
Fan No Fan
Output (kW) 16.4 - 22.3
Manufactured From/To Unknown - 1983
SAP seasonal efficiency 65%
There are sites on the web that might sell you a manual but its age is against you I suspect.
It may be due for a service - the engineer might know how it operates if he's long in the tooth.
Good luck...
Peccavi
September 2009