Metallic Crystals

1.45 describe a metal as a giant structure of positive ions surrounded by a sea of delocalised electrons

That's what a metal is, a lattice of positive ions in a sea of delocalised electrons. Basically delocalised electrons is as the name suggests, it's not attached to any particular atom and is able to move freely. 

1.46 explain the malleability and electrical conductivity of a metal in terms of its structure and bonding

Metals can conduct electricity because the delocalised electrons are free to move and carry the charge. The key words here are that the electrons are free to move/mobile and can move through the structure. 

Metals are malleable and ductile because they are a lattice structure and arranged in layers, which are able to slide off each other easily when a force is applied. [The layers are the sheets of positive ions.] Thus making it easy to bend and shape-->malleable. Easily pulled out into wires-->ductile.  

Metals can be made harder by alloying them with other metals. An alloy is a mixture of metals-for example, brass is a mixture of copper and zinc. In an alloy, the different metals have slightly different sized atoms. This breaks up the regular arrangement and makes it more difficult for the layers to slide. 

a pure metal, with neat layers that would easily slide over each other

the atoms have different sizes, so it disrupts the regular packing and makes it much more difficult for particles to slide over each other when a force is applied. this tends to make alloys harder than the individual metals that make them up. 

A common example of an alloy is stainless steel, which is iron mixed with chromium and nickel. The chromium and nickel form strong oxide layers which protect the iron. This is why stainless steel is so resistant to corrosion. Obvious uses include kitchen sinks, saucepans, knives and forks and gardening tools. But there are also major uses for it in the brewing, dairy and chemical industries where corrosion-resistant vessels are essential.

I'm sure you all know the difference between elements, compounds and mixture by now. So I'd just like to point out that alloys are considered mixtures rather than a compounds because of the totally variable proportions of the metals. Whereas in water (a compound), every single water molecule has two hydrogen atoms combined with one oxygen atom. In the alloys, the metals can be mixed in any proportions...