Right, this is the story from Colindale.
Auto does not mention either meeting.
Motor Cycling reports on the meeting on the 7 April; but not the two on 9 April
Motor Cycle reports on the two meetings on 9 April but not on 7 April.
The local papers, West Essex Gazette and Loughton Advertiser, both report the meetings on 9 April but not the meeting on 7 April. However, there is a line in the Loughton Advertiser report which says that the meeting on 9 April was the third meeting held at High Beech, which indicates there was a meeting on 7 April.
So, I think we can safely say there was a meeting on 7 April; but it seems strange that only Motor Cycling reported it.
Also, in the course of this investigation, I came across the report on the meeting at Greenford held on 7 April as well, also in Motor Cycling. I could be persuaded that, in fact, this was the first "real" speedway meeting in this country. There is an accompanying photograph which shows two riders, one of whom is Billy Galloway, the other unnamed, with their feet on the track cornering. Curiously, Galloway appears to have a foot forward style. (The other is leg trailing).