Sillywalks.
If an a/c requests start before the EOBT +15 period has elapsed and is then delayed by ATC, it is ATC's responsibility to keep the EOBT upto date.
http://www.caa.co.uk/application.asp...detail&id=1392.
At Gatwick this will be done routinely for you. I can't speak for other units.
Hope this clears it up.