House Martins Building a Nest Timelapse - Summer 2014 (not Swallows)

Here's a short timelapse film of House Martins building a nest on the eaves of a house.
I originally thought they were Swallows but they do not have the familiar red patch.
I was sure that birds built their nest in about a week but not in this case!
I can only guess why it took so long and why they decided to change the construction of the nest after a few days.
The weather conditions were very cold in the first few days and hardly any progress was made.The House Martins left the mud shelf that they had built at night to roost in a warmer place I think.
As soon as the sun came out and the temperature increased, they made much more progress particularly after a downpour. Maybe the mud was easier to find. They make over 1000 visits to build the nest using as many mud pellets mixed with their saliva.
Why did they change the nest design? Maybe another pair bullied their way in or just one of the adults!
Why do they make the nest over a vent? Maybe it's the warmth from the house!
I would love to know why!
Some techy information for you. It took 2665 photos at 4 minute intervals over 16 days during daylight hours.
Post edited in photoshop first then created the movie in Pinnacle 16.
I have to thank R6angie, whose house I I used to shoot the timelapse sequence. She did most of the hard work getting up at 6.30am each morning to turn the camera timer on. Also daily camera battery changes and 4 memory card changes. She isn't technical so I'm amazed she stuck at it encountering many glitches throughout the 2 weeks
I hope you enjoy the timelapse film.
I would be very interested to know your thoughts on why it took the House Martins so long to build the nest and why the design was changed half way through.
Thanks.