Dear Maybird,
I'm not that familiar with the Codefree B/G monitor, but someone will probably correct this, but I would think that the additional 'spare check test strip' that you are refering to is something that you have been supplied with is something that can be used with to check the calibration accuracy of your test meter. This is something that many manufactures supply, and I would suggest that to confirm if this is the case, then you contact the manufacturer's web site and/or send them a suitable Email.
Don't worry about the minor difficulties that you have in using your B/G meter at this stage, it's something we all get through and will come automatically to you once you've done it a few more times. The 5.6 mmol/L morning reading is good news.
As for the blood pressure tablets, well I've been on a variety of differing types over many years and have never found any to adversly raise or lower my own B/G readings.
Best wishes - Lazybones