Hi KiethI had the same experience when I drove my 98 from England to Scotland. The car was fine at motorway speeds but on tickover it was as rough as a Bears A~~~ ,turned out the battery was completly dubbed.The electrical system for the engine will take its primary power from the battery and if there is insufficient amperege in the battery it will not service the engine requirements efficiently, when the Battery is failing basically the alternator is running the car which is not a good thing This is the time of year that batteries fail most when they are asked to perform and are at the end of their life,they literally have a heart attack.
So everyone should check the state of their battery before it lets you down.There is a tool at some quality workshops that will give a reliable reading of the battery state .Tell tale signs are interior light brightens and dims on increased engine revs .
A simple test can be carried out at home just run the battery right down ,take it off the car and connect it too an old headlight from the scrapyard, then charge it up on a normal battery charger if it takes the charge well and charges up at a good rate then stays charged up it is in good shape ,if it loses the charge or takes a long time charging it is on its way out.
cant really see someone doing that simple battery test you mention to be honest? unless I have read it wrong??