Issues with second monitor on laptop via HDMI

[Metal Mick]

I have a shiny new Mint 17.3 install. Kernel is 4.2.0-36. My machine is an Alienware R3 17" laptop with the nVidia 970M card. Drivers are the recommended 352 as per the Driver Manager. I tend to use the machine as a desktop replacement, and find my larger monitor (Dell 29") gives me better screen real estate.

When I connect the monitor via HDMI, usually, but not always the machine boots into Mint setting my screen resolution to 1920 x 1080, instead of the 2560 x 1080.

Checking display preferences, I find the display has "mirrored" checked; I can uncheck this and resolution remains unchanged; setting the monitor resolution to its default usually results in it remaining at 1920. But not always. It's just a pain repeating this process until it finally sticks. Many times, both displays go blank and my external monitor displays a message telling me, "The current input timing is not supported by the monitor display..."

My laptop has a combination Thunderbolt/mini-DP connector, and I'm waiting for an adaptor to arrive so I can use the displayport cable, but it could be the week after next before it arrives. (My previous laptop had the same setup, but with a min-DP to connect the monitor, and all was just perfect.)

Is anyone able to suggest a way I can get my monitor display to remain at the correct settings, hopefully from boot? I'd really appreciate it.