Garmin express downloading files time remaining increasing

I have just purchased a Nuvi 2599 LMT-D, and have been trying to update it using Garmin Express on my Windows 8.1 PC. The Express software installed on the PC with no problems, and upon connecting my Nuvi, it was recognised immediately. Express told me there were software and map updates available, so I instructed it to install all, and it started downloading, but very, very slowly. I ended up leaving it on all night, but when I came back to it in the morning, it said the updates had failed to install. There was no further information to indicate why this happened. I therefore tried again, and despite saying initially that it might take 3 hours or so, it has been going that long with little progress (about 3%), and the estimate keeps increasing (currently 12 hrs and increasing). It currently shows a download speed of around 27 Kbps. This is on a reasonably fast broadband connection which I've tested (with Express closed down) and found to be rated at the usual 3.5 to 4 Mbps. I've also tested other downloads and streaming, and these are fine whilst Express isn't running. As soon as I launch Express and start the update, with nothing else downloading or streaming, the overall broadband speed slows considerably, but very little progress is made. To reiterate, the problem only manifests when Garmin Express is trying to download updates. Before my latest attempt to install the map updates, I did try installing the smaller updates first (i.e. traffic, keyboard etc), and these appeared to download and install extremely quickly, with a green banner indicating success, but then when I quit and restarted Express, it showed the exact same updates as still being required. As such, I decided just to try the map update on its own, and that is what is now running incredibly slowly, currently 16.4KB/s and 3.4% complete.