Mate,
I'd just go for it. I put off applying for nearly 10 years for a few "mistakes" I made in the past. Nothing too serious but a few arrests for non violent (drunk and disordalies and a caution for threatening behaviour). I also had a course of beta blockers after a family death, i didn't mention this as I still view it as a bit of a rough couple of months rather than depression / medical.
The firearms officer couldn't have been a nicer guy, I told him i'd been a "bit of a tit" a few years ago and he said he'd seen alot worse. He issued me a cert. within a week no problem. He didn'