Thanks for all your interesting and thoughtful replies. They are much appreciated.
In answer to my own question, I tend to elongate my meditation session if I feel it is taking me longer than usual to settle into the groove and achieve at least a measure of stillness.
Last night our Worcester 28i Junior central heating boiler packed in. Again. The thing is a an abomination and has caused us nothing but trouble, along with endless call-outs to service engineers, over the last fifteen years. I know every widget...every bolt and sprocket of its internal workings, due to the fact that it has gone wrong
so many times.

I spent the usual unsuccessful half-an-hour fiddling with it either side of midnight before settling down to fitful sleep in a stone-cold bed. I slept poorly and woke up frustrated, agitated and gently fuming to myself.
Why am I telling you all this? Well, as a consequence, my morning meditation session started with a succession of intrusive, angry thoughts, almost entirely directed towards my central heating boiler, along with a febrile, scrolling menu of options regarding who I was going to ring to come and fix it and how long I should hold out before I rang them. Sometimes sitting there with your eyes closed to no great purpose just invites this kind of introspection and analysis.
I have, however, leant to be patient and not 'write off' a meditation session. If I keep applying myself gently to the idea of calming the mind, eventually it will start to happen. It may take a couple of indifferent and rather distracted body scans, but it always seems to happen that eventually I start to settle, gain focus and calm down.
The whole, lousy central-heating dilemma didn't go away, it just 'dialled down' in intensity until it became distant, disconnected, shapeless...somehow
separate. The stillness was the stillness and my Worcester 28i Junior central heating boiler existed in its own parallel universe. I had total respite from the negative thoughts which it produced and the feelings which accompanied it...
...so this morning I mediated for 30 minutes instead of the usual 15-20 because I needed to. I ended up in the same place; it just took a little longer.