Had a look around this time and nothing obvious seems to be showing for Wembley Central (not entirely surprising for an engineers' train though).
The only freight working through the station I can find is 2x EWS 66s (66094/66182) at c.7am in the morning running from Wembley Central to Wembley EFOC. Given it was 2x 66s and a path for a 5 min journey, there's a chance this could've been an engineers' working and these have been operating in the area later in the day. However, with EWS-liveried 66s, could be any one of hundreds!