What I do on a flat lockout
Flats are often a two-door problem: a main communal entrance, then your own flat door. Depending on where the problem is, the approach changes.
Your flat door is usually a single Yale-style night latch or, in newer builds, a Euro cylinder. Both pick open non-destructively most of the time. On older conversions around Stoke Newington, Finsbury Park and Crouch End I often see a pair, a Yale night latch plus a 5-lever mortice deadlock, and both have to be dealt with, but neither usually needs drilling.
The communal entrance is usually a Euro cylinder with a snib on the inside, often worn down from years of 30+ people using it. Sometimes the cylinder has been replaced with a snap-prone budget unit that a locksmith can pick open quickly. If you’ve lost the fob or key to the main door as well as your flat, I can address both on one visit.
Leasehold quirks: if the cylinder has to be replaced, I’ll cut you an extra key so you can give one to the freeholder or managing agent, they usually want a record of any lock change. I won’t notify them directly; that’s your call.
Newer builds often have a smart lock on the flat door (Yale Linus, August, that sort of thing). If it’s a battery-dead issue rather than a hardware fault, that’s often a 30-second fix with a fresh battery, not a lockout job, describe it on the phone and I’ll tell you if it’s worth calling out.