Please find set out below references and clauses from BS 5628-3 2005 Code of Practice for the use of masonry – Part 3: Materials and components, design and workmanship.
Clause A.4.1.3.3 Stone
Deliver the stone from the yard to a suitable off-loading facility on site, and in the fixing sequence. Stack clear of the ground on battens to prevent contamination from moisture and soluble salts in the earth. In wintry weather take precautions to prevent damage to the stones from the freezing of rainwater or residual quarry-sap by covering with tarpaulins or polythene sheet over straw, Hessian or other suitable insulating materials, which contain nothing that might damage or stain the stone. Protect against staining from other building materials, particularly hardwoods, oils and fuels.
Clause A.4.2.3.8 Mixing in cold weather
Do not mix mortar when the temperature is at or below 3ºC and falling, or until it is no less than 1ºC and rising. Do not use fine aggregate (sand) or semi-finished mortar (lime-sand mixtures) containing ice particles.
Clause A.5 Brick and block walling
A.5.1.1.1 Cold Weather
In cold weather do not:
Build masonry when air temperature is at or below 3ºC and falling or unless it is at least 1ºC and rising.
Lay mortar on frozen surfaces
Use wet bricks or blocks when there is a danger of freezing.
Clause A.5.1.1.2 Frost damage
If the mortar is susceptible to frost damage, obtain guidance and instructions before proceeding work.
Clause A.5.1.1.3 Protection
Cover the tops of newly-built brick and block masonry to protect it from rain and also from frost, if imminent, and at all times when work is not proceeding. If there is any danger of the work being frozen then consideration should be given to the use of insulation under the cover.
COMMENTARY: Unless the work is protected when not proceeding there is always the risk of sudden frosts or showers causing damage. It is important to cover work at the end of each day. These covers should be kept handy for use. Wet covers can freeze in contact with the work. It may therefore be preferable to top the wall with a clean and dry wooden plank, wider than the wall, and place polythene or similar sheeting over the plank, clear of the wall face, in order to provide an insulating air gap. It is important to weight the covers to prevent the wind lifting them.