Hi All, So, so grateful for all replies and will get back to them later...Have just collected wife from hospital and needs little R&R...an in own car, no taxi! I figured out what was the problem...the car battery booster (mobile phone size gadget referred to in OP) has a little bulb on side and I had only just figured this out today...it flashes Red, Red/Green, & Green. It also has a little 'click' switch. Anyway, gave it 1 last go, made sure all connections correct, also confirming Green light on (on the part that has red black crocodile clips that plugs into booster) and gave ignition switch a turn...and what happens...it bloody well starts!
Just 1 piece, I also managed to find the magic eye which shows-looks like it shows-green in a clear circle, which on the instructions on battery says...Recharge! So gave car a blast, 8 - 10 miles, and got call to collect wife.
Normally we do drive enough miles per week to stop this from happening as no problems before lockdowns etc., but since, have only been doing very short daily drives 5 or 6 days pr week., which aren't probably even 2 miles & know this doesn't do battery much good.
So, all good for moment, and will see what Monday brings, either leave as is or local garage for new battery! Will get back to you people later. Thank you so, so much for all your replies.