I am getting ready to buy in the cabling to have electricity in the house. The plan (as discussed in an earlier thread) is to leave the meter box at the site boundary until the garage is  completed and run SWA (from a switched fuse that will replace the existing site consumer unit) to the house. This will be approximately 20m distance.  Looking things up in the blue book, it looks like I need 25mm^2 SWA if I want a 100A feed to the house on the assumption that approximately 15% safety margin is sufficient. The voltage drop looks to be insignificant compared to the requirement at this distance at full load. The other alternative is 35mm^2 but this feels like overkill and it probably has the same bend radius as my Range Rover's turning circle! Thanks.

The likelihood is, because of the distance, you will have to de-rate to an 80a fuse at the point of origin. Based on that, a 25mm SWA will more than suffice.

I wouldn't get too hung up on having a 100a supply, as an 80a supply will give you more than enough 'juice' for a typical residential dwelling. 

I would recommend you use a switch fuse with an 80A fuse in it.  Then in the unlikely event of the fuse going, there is a reasonable chance it will be your 80A fuse that blows and you can easily change it.  If the DNO's 100A fuse goes, expect them to ask some very awkward questions about what you have been doing to blow it before they will replace it.

 

Over that distance and with an an 80A fuse, 16mm would do, but agreed 25mm would be better.

 

Leave it a bit over length at the meter box end to allow some room to reconfigure things when you build the garage.