Which variety of silver birch are they, specifically?

Am also interested in what variety the sold you. You can make a multi-stemmed birch by buying a sapling and removing the leading stem, allowing the lateral branches to grow and eventually form a multi-stemmed tree. Himalayan Birch is best variety for that. Have not seen it work well on native UK Betula's!, hence I am interested to know what they sold you.

And that's why I asked as well, Kildermorie - jacquemonti particularly tends to form 3 or 4 stems as it grows quite naturally anyway.

It does seem to work on the native B.pendula. I am thinking of forestry situations where the trees are deliberatly copiced for firewood etc.

It worked well on the self sown Birch which we have in the garden. All I did was remove the growing point and left a stump of about 12 inches high. Not as pretty as the white barked varieties but it looks good enough where it is.

I reckon the VERY FIRST thing you should do is take them out of the bucket of water. They can have a dip for 6-8 hours if you can't plant them immediately, but most tree roots will rot off if you leave them in there for long.
See http://www.ashridgetrees.co.uk/Planting-Bare-Root-Trees
For planting advice and care of bare root trees if you can't plant them immediately - if the ground is frozen, for instance.
http://www.ashridgetrees.co.uk/Cold-Freezing-Weather