Confirming the original manufacturer of your bathroom helps to identify the item you require. But
it's not essential.
What to look for
Have a cursory look at your suite to see if you can spot any names or logos. You
don't need a microscope to do this!!
You are looking for easy to read
details put there by the manufacturer to advertise themselves.
Where to look
A good place to look is under the china cistern lid of the toilet. This assumes
your WC does have an simple to remove china lid. Simply lift it off (don't drop
it) and have a look underneath.
Make a note of any names or numbers. If it's
small and hard to read it's probably not important.