@ghawkins @neil
To be fair, even before playing with a MiFi+Smarty, aka Three, data SIM I knew a better solution was required.

Got a ex-Three ZTE MF286D router and
a Poynting 4G XPOL-2 antenna off eBay.
The MF286D & its two lollipop stick antenna was only marginally better than the MiFi.

Attaching the XPOL-2 was the deal, 10x improvement in thruput (speedtest.net & iPerf3). Circa 60Mb/s now, and that may be improved by proper antenna mounting rather than balanced on internal window sill.

Quite pleased for approx £90 eqpmt outlay and £20 SIM monthly recurring.

Just for good measure, I’ll try for an omni antenna to establish the difference in performance. My particular requirement is fixed but have a few non-tech friends with camper vans & caravans who may benefit.