Nigerian Airlines were evicted from their offices in Terminal 3 South Wing Building so an empty slot there and remember Gulf Air get an extra flight in summer too bumping it up from 4 flights to 5 flights from LHR - GF002, GF004, GF006, GF008 and the new flight GF012.
Qatar airways I think is also increasing their frequency from LHR as is Etihad after their inital startup.
They have reopened Gate 23 in T3 which was closed for pier reconstruction (I think) and to tell you the truth, most of the extra flights which are coming in are night flights, going out of LHR.
Only half the number of Gates at 2130 to 2300 are usually full with A/C. For example Gulf Air run the GF6 at 2030 and GF8 at 2200 with an extra slot for 2235 from the 28th March. Etihad's single flight days depart 2115 and Qatar Airways (I think) is an evening flight.