Advice on buying & installing nest boxes

My understanding is that all nesting boxes should face north so that the box does not get overheated...

The RSPB website is a good source of information for answers to your questions.

The only success we've had with blue tits is on north facing walls, they get too hot facing the sun.

To be honest we never put up nest boxes... plenty of hedges and other natural environments for the birds to nest in.

Aside from hole size, spacing is the most important thing in placing nesting boxes. Too close and you will have constant fighting amongst different and same species and if you have a bird feeder and or bird bath, keep them well away from the boxes. I have had great success in getting bluebirds (USA) to nest in my garden. They need about 1/2 acre around the box for territory...this is one of the reasons to pay attention to spacing requirements. All of my boxes face east. My reasoning for this is to give them a quick warm-up in the cool morning.

Height makes a difference too - different birds like to nest at different heights - RSPB will have info on this as well as the boxes.

I'd definitely go to the RSPB website for advice. What I will say it that I have made several nest boxes over the years and have installed them to the heights that certain bird species should like. Having said that, I have never had blue tits nest in my "suitable for blue tits" nest box (following instructions from RSPB carefully) but I have had blue tits nest for the last ten years, in the middle compartment in the sparrow terrace I put up! The birds will make their own choices.
I would advise that nest boxes with perches are not suitable at all. I can imagine a hungry predator using a perch to make it easier to take eggs or hatchlings.

Thanks for your answers everyone.

I've now installed the Blue Tit box on the side of my shed and may put some prikka-strip, trellis or similar above to deter cats from the roof.

Will be putting the Starling box in the tree and a Blackbird Open Nest Box on the fence soon and after looking at the advice online have decided to put a Schwegler Woodcrete Avianex on the side of my house instead for Great Tits or House Sparrows.

Will be interesting to see if anything gets used next year!

We have loads of boxes up and several still in the shed which didn't get put up! Apart from making sure that the box is not in a position that will not 'cook' baby birds during the day, I'm not sure it's absolutely vital which way it faces.
Keep meaning to make and fit a sparrow terrace on the same principle as one I made in the days I kept budgerigars. This was a long straight box (simpler to make) with all the holes facing the same way and removable roof.
So that they knew which hole was their own, they were numbered in different colours! I know birds can't count, but the colours or shapes seemed to work so that they knew.
Now I know that not everyone would want a row of numbered boxes for the sparrows but a motif or any shape would do.
Best of luck with your boxes, our native birds need all the help they can get these days.