The strake mod made it out to MEAF in 1960 so that was probably a year or two after the rest of the RAF.
May 25th 1960 in WZ884 I, accompanied by a young J T S Lewis, did the post mod airtest. Don't think we found much difference.
The only problem was that F.O. Neil Williams complained he could only get a 360 flick on the top of a loop rather than 540 pre-mod. Probably a good thing as the top wing surface went rather crinkly doing that.