A quick-and-dirty analysis of what's been into LHR during the past week, where the airline in question didn't use that type to LHR in the first week of March (or didn't operate to LHR at all) brings up the following:
ABW B744
AFL B77W
AJT B763
AWC A318
AZA A332
BCS A332
CCA A332
CES B77W
CFE AT45
CPA A35K
CSN A388
CSN B77W
CYP A319
ETD B77L
ETH B788
ETH B789
HFM A319
ICE B738
IRA A332
ISR A320
JAL B789
KZR A21N
MEA A332
MGX E195
MSC A320
MSR B789
PVG B763
QTR B77L
RJA A310
SAA A346
SIA A359
SVA B789
SWR A20N
TAP E195
UAE B77W
UAL B78X
A few of the above may be due simply to the Summer schedule change, but most are a result of ad-hoc, repatriation and other non-standard flights.