DavecUK
Righty-ho. I bit more time today to take a more methodical look at the swing in pressure and also the timing.
Firstly, I left the machine to warm up for about ⅓ hour in case I was losing excessive heat through the group-head and other pipework.
Pressure-stat kicks in at .9bar and stops at 1.2bar (give or take as I am reading the measurements from the analogue dial). It then takes around 1:40 to return to .9bar when the pressure-stat kicks in again. This was consistent across three readings.
If this is a normal range and timing, then perhaps I was a little hasty the other day in thinking there was an issue (in truth, I was just stoked that the machine seemed to be otherwise operating as before!)