You MUST
Night (the hours of darkness) is defined as the period between half an hour after sunset and half an hour before sunrise.
External links: Laws RVLR regs 3, 24, & 25, (In Scotland – RTRA 1984 sect 82 (as amended by NRSWA, para 59 of sched 8))
You MUST NOT
In stationary queues of traffic, drivers should apply the parking brake and, once the following traffic has stopped, take their foot off the footbrake to deactivate the vehicle brake lights. This will minimise glare to road users behind until the traffic moves again.
External link: Law RVLR reg 27
You should also
Hazard warning lights. These may be used when your vehicle is stationary, to warn that it is temporarily obstructing traffic. Never use them as an excuse for dangerous or illegal parking. You MUST NOT use hazard warning lights while driving or being towed unless you are on a motorway or unrestricted dual carriageway and you need to warn drivers behind you of a hazard or obstruction ahead. Only use them for long enough to ensure that your warning has been observed.
External link: Law RVLR reg 27