Officially at least, the information you've received seems to be completely wrong.
As you're probably already aware, Romania is NOT a Schengen country.
They do however have special provisions to allow people with a Schengen Visa to transit through the country for up to 5 days, but only where the intent of the travel is for transit to a third country.
In your case, you will not be transiting - you'll be entering and leaving to/from the same country (England) and on flights to/from the same country (Frankfurt). What's more, the country you'll be coming from and going to (England) are NOT Schengen countries either, which likely breaks the rules of the "transit" provisions too.
I would suggest talking to the embassy again and point out your exact travel plans, and that you will not enter/leave Romania from/to a Schengen country.
Update: Here's the official answer from the Romanian Ministry of Foreign Affairs. The relevant paragraph is :
In order for a uniform visa, a national visa, or a residence permit to
be equivalent to the Romanian transit visa, under the conditions
listed above, it is imperative that the transit through the territory
of Romania naturally fit into a coherent itinerary to the country that
issued that visa or residence permit, or into a return trip to their
home country or country of residence.
In your case, none of those conditions are valid, and thus your Schengen visa almost certainly isn't valid for entry into Romania.