Thanks @WieWaldi and @rsl12 - it's very kind of you to look into this on my behalf. All that info will be very handy if I decide to use a cable to connect my piano to the tablet, but for now I've put the tablet to one side (to prevent myself from smashing it to pieces with a hammer 😬).
Over the past couple of days I have been experimenting with using ways to prevent the unwanted reverb/white noise sounds. I placed some rugs/blankets under the piano (including the legs) and the tripod - this made no difference. I removed the tablet from the tripod and put a layer of material at the back of it, so that it fitted more snugly into the holder - again no difference. I removed the tablet and held it in one hand while playing a few base notes - this reduced the unwanted fuzzy reverb a bit, but is hardly a practical solution as I'd need to play everything with one hand! (Although there is a one handed concert pianist who recently performed at the London Proms - he was amazing!)
Today I reverted back to using my Chromebook and managed to get a reasonable recording done. The picture quality is not as good as the tablet and I cannot get the nice camera angle that the tripod could provide (as the Chromebook is too large to fit on the tripod), but the sound quality is better. I think perhaps the tablet is just much more sensitive to vibration than the Chromebook, which has a more substantial case to shield all the delicate components? Or perhaps the tablet's microphone is just a much higher spec - which probably works well for recording the human voice, but does not work well for electronic instruments? Anyway, I can use the tablet for other things, it takes good quality photos and I have a beautiful great nephew who is very photogenic (because he is actually the BBITW - Best Baby In The World) so the tablet will get plenty of use when I next visit him. 🙂
