Easiest way is to use the Seagull Engine Identifier.
Go to http://www.saving-old-seagulls.co.uk
Click on “British Seagull Engine Identifier”
Enter your Serial Number
Hit return and you should get this;
Unfortunately in this case, the manufacture date does not appear. However in most cases, the month and year of manufacture are clearly advised.
Might be worth double-checking that serial number and trying again.
Hope the above helps.
Regards,
Spiro