Well, whaddya know?
Success!
I unexpectedly had some spare time so put it to good use as follows:
Rather than use the shielded USB cable that I had in mind I pressed a previously redundant Belkin USB external antenna cable and mount in to service together with the previously internal original MX200i antenna. I did this because the length was the same as the heavily shielded cable but it has the advantage of stand alone vertical mounting if I needed it (and if not, simply unclip the base from the USB connector and use horizontally).
Here's what it looks like inside the case:
I routed the external cable through an existing hole in the case for those models with ethernet connectivity that mine doesn't have. The hole was previously covered by the black sticker to the left of the case. I will now plug it with a rectanular rubber grommet that I have in my "box of bits".
Finally, here's what the antenna now looks like.
Simply moving the antenna externally from the case made an average 5% signal improvement from multiple tests when I taped it without the base to the back of the MX200i. Mounting it vertically and being able to chose location added another 10-15%, so 15-20% in all. All this was in my home office. When I took it out to the shed though I was in for a shock... What was previously a weak 10-15% connection (and sometimes 0%) to the Home Hub was now 55-65%, largely depending on where I was stood. If I walked as far away as I could still read the WiFi strength through the open shed door the signal was still 40-55%. Absolutely no disconnections or rebuffers either, whereas switching to the alternative WLAN gave three in the first five minutes despite now having 85-95% signal strength courtesy of the antenna modification.
So, problem solved, at zero additional expense, and I will now tidily encase the antenna in polyethylene as well as plug the hole as mentioned.
Happy camper
Great minds think alike, thanks, Bushfiva.