These are cheap and light, and readily available. However they are usually a poor choice since they are often too short to get adequate leverage, they are also usually a somewhat sloppy fit which risks damaging the nut and also injuring yourself should the spanner slip:

Again, a clean square cut is important, and any burrs removed. Take great care to protect pipe ends when pulling them under floors etc, since its very easy to scratch plastic pipe and this makes it hard to get a good compression (or pushfit for that matter) seal. Sometimes an olive can be removed by tightening an adjustable spanner closely around the pipe, and then using it as a slide hammer to strike the nut or the olive itself and drive it toward the pipe end.Also means that you have twice the number of positions that the spanner can engage with the nut. This is very handy when there is very limited turning space for the spanner. They are very skinny which makes them uncomfortable to use as well, but sometimes this slimness is an advantage where there is not much space. One of the difficulties with compression fittings is that while the nuts are usually a nominal size, there is not agreed standard tolerance between manufacturers. This can make selecting a single universal spanner difficult. Also not all spanner types can be used in all situations. Hence most plumbing kits will have a number of spanners, including at least one adjustable one. The pipe needs to be cut square and cleanly, with any burrs removed. Pipes cut with a pipe cutter are much easier to use than those cut with a saw. The pipe needs to be clean and free from contaminants like paint or plaster etc. It does not need to be quite as clean and shiny as required for soldered connections. Copper and brass olives both work fine. Olives can't be removed from a pipe and reused. Even hand tightening a fitting with an olive inside before the pipe is inserted, is enough to make the olive unusable.

The most effective "cure all" sealants are the silicone based "setting" sealants like Fernox LS-X. These will usually seal most problem joints so long as the olive is actually meeting the fitting body when the nut is tightened.Available in three sizes, 16mm – 40mm, 40mm – 75mm and 63mm – 125mm. This compact tool is a necessity for all your compression installations and repairs. PTFE tape can also be used as a sealant - a few turns need to be wound over the olive such that they will end up sat between the mating surface between the olive and the fitting: These are far better suited, are long enough to get good leverage and are comfortable to use. The design of the end wraps around more of the nut giving better support and greatly reducing the risk of damaging a nut: The partially "open" end allows the spanner to slip over a pipe, and then slid onto the nut. Using a spanner, tighten the nut over the threaded head of the fitting, while holding the fitting with a pair of plumber’s grips. This pressure created by the tightening motion will press down on the olive, causing it to grip the pipe tightly. However, be careful not to exert too much pressure as this could damage the fitting or thread, making the device harder to remove or reposition