Got up this morning determined to find a solution.
The last part of this post is a mystery to me - but first:
Having to assume huge aerial is faulty. 5db one not as good. Back to the little one.
To test the local signal, I put the second NETGEAR v 3 unit in the plastic conservatory that's fixed to the back of the garage. To my astonishment, I'm now getting the strongest signal for a long time from the FIRST and active unit. Coincidence? Probably.
The borrowed second unit giving a modest two bars.
Haven't co-related car in or out of garage - good thinking Bat Man.
Mystery bit: My pal once put a yellow wire into the conservatory. He says he simply connected the Linksys one to his NETGEAR via a yellow wire. OUTPUT TO OUTPUT. This can't be right . . . can it?
It's my intention today to take a 'snapshot' of his settings, then use mine as a relay station. It means pal and family must not need the net during my messing about time as it seems to be a lengthy procedure. Well, not if things go well, but in the real world...