tophat - I agree that website
Flights within Europe - Flight offers - Book flights online now has an English language version, but the airline has 2 public facing websites - which site you use seems to depend on the route you want to book.
The website which allows for booking to/from Southend,
OLT Express GMBH - bei OLT Express jeder Tarif kann online gebucht werden. alas does not (yet) have an English-language feature
OLT flights from Southend seem to begin 2 weeks from today