"Passed through T1 this morning for an easyJet flight to Budapest, we're being held on the ground until 06:30 due to "atc restrictions"
I wasn't aware of any issues, can anyone shed any light?"
Assuming it was a 'slot delay' it would most likely be due to too many flights planning to go through a certain ATC sector, or arrive at the airport, at the same time. Happens daily, like irishlad says. Any further issues such as weather (thunderstorms, fog etc) or staff shortages would reduce capacity and lead to bigger delays. I'm guessing you weren't due to leave until 0600 or after (UK first wave departures don't really start until then) so that delay doesn't sound excessive - probably just smoothing traffic levels through en-route sectors.