I am another european using the Seven Plus and taking insulin shots. While it’s true that a lot of people on this forum seem to be using a pump, worry not as you’re not alone!
On the calibration issue, my receiver sometimes lights up at night asking for calibration, but it does not vibrate, and after a few seconds goes back to sleep. If this happens while I sleep, well… I keep on sleeping! Not sure why yours vibrates, on mine I cannot see any specific setting that would modify the behaviour to silent mode.
Make sure you’ve calibrated your sensor properly, by choosing to install and calibrate during times of flat readings around the 120 mark, which happens to be the default figure when the receiver asks for the data input at the end of the 2 hours initial calibration window.
On the accuracy of the data, it is quite precise but do remember that blood-derived BG readings will always be quicker to be spotted compared to interstitial fluid BG readings. My experience is that the lag can be anywhere from 15 to 20 or even 25 minutes. The Dexcom box knows about the delay, of course, and tries to interpolate as much as possible. 3 points diff is sweet, and it’s way within tolerance for both the fingerstick and the Dexcom. Fingerstick readings can be within +/- 15% too, so don’t necessarily trust the fingerstick reading. But of course since it’s a blood-derived reading, you’re going to read a figure that you will read on the Dexcom in, say, 15 minutes. And that is why you want to calibrate during a nice and level BG state.
On your last question, you should see the devices that were the previous generation to this one, and then you won’t complain any longer! I’ve done a 3 days trial with an early 1st-gen device which had the bulky sensor connected by a short cable to an even bulkier battery pack… ouch!
In a perfect world I’d like to have a sensor under my skin, without the need to change it every week. Maybe we’ll see this happen in a few years, who knows?
P.S. Is your Dexcom calibrated in mmol/l? I don’t have this problem as in Italy we use mg/dl like in the US, luckily, but I was curious to see if the Dexcom has been localized for the UK.