There we go:
The reason why radiators are mostly fitted here is to counteract downdraughts. These downdraughts are not draughts from outside - you will have downdraughts even with the most airtight windows. Downdraughts are the result of room air being rapidly cooled on the cold surface of the glass, causing the air to fall and producing a draught. If it were true that radiator shelves direct warm air from radiators to the centre of the room, it would be equally true that shelves direct cold downdraughts to the centre of the room. In fact, neither of these statements is true.
It is also untrue that heat from radiators is immediately lost through windows. This will only occur if the window is open. Radiators are mostly installed under windows because this is generally the preferred position for them.
As ANY radiator manufacturer will confirm, radiator shelves actually REDUCE the heat output from your radiators, typically by around 5%. Therefore, this is hardly a measure to increase your comfort ? or to save energy. Avoid radiator shelves.
'Always tuck in curtains behind the radiator when it's dark'
It's sound advice to close curtains at dusk to help reduce heat loss. However, tucking in curtains behind the radiators is not recommended.
With most modern radiators, at least half of the heat output is from behind the front panel. If you tuck in curtains behind the radiator, you will restrict the free flow of warm air. In some cases, tucked-in curtains will completely block this passage of warm air, and substantially reduce the heat output. Therefore, the best advice is not to do anything that restricts the free flow of air around the radiators, or you will reduce heat output. This also applies to space under and in front of the radiators, where there should be a minimum of 4 inches clear.
If your curtains drape below the top of the radiator, you must consider buying shorter ones. If you are not prepared to do this, consider tucking the curtains onto the window sill when it gets dark.
If the tops of your radiators are very close to the underside of the window sills, ensure your curtains are as short as possible. When drawing curtains at dusk, tuck them onto the window sill. If you're not prepared to do this, it may be better to hang them loose and in front of the radiator, rather than tucking them behind, as this can cause less of a restriction to the free passage of warm air. However, if you have a pelmet above your curtains, then leaving them loose and in front of the radiator is not advised. You'll need to use a bit of judgement. Allow plenty of space for air to circulate freely around radiators.