Please can you tell me where is the best place to site a new wooden open fronted blackbird nest box within a garden.
There are a few things to consider, and you need to adapt these to fit the available sites in your garden.
If you can, try several positions - remove the ones that don't seem attractive to the birds, and reposition.
Orientation - books say anywhere from north-east through east to south-east. The entrance itself needs to be sheltered, usually an overhang of the roof is good enough.
Height - about 2m minimum (though some people who put in old teapots, spout downwards, for robins, say lower is better).
The entrance needs to be positioned such that the bird can have it's wings fully outstretched to fly out and in. A "staging post" about 2m away (can be natural, such as a fence or branch) helps. Don't put one on the box - it is too handy for predators.
Position boxes away from feeders, otherwise nesters will wear themselves out trying to drive off the feeding birds. Likewise, boxes need to be at a distance from other nest sites.
Covering with ivy or other vegetation can help with the camouflage.