I got the same email too, so emailed back saying they hadn't actually answered my question about zip top bags. I then received a reply saying that if my plastic bag was within the DfT measurements, it was ok. I again wrote back asking them to confirm zip top bags are ok, and the measurements they allow, but sadly, they don't seem to want to put that in writing.
The free one they issued me was 9" x 6" (DfT say no more than 8" x 8") My zip top bag is 7" x 4".
Oh well, the next time I go through the crew security check point should be fun.
My daughter tells me that Eamon Holmes has just written an article for a newspaper saying that his wife was told at Belfast airport that her "airport approved zip top bag" is no longer allowed. The madness is spreading.
just carry the literature from the DFT, thats what i do.