As Mike's been finding lately, trying to sleep in the daytime just isn't the same. After 90 minutes of lying in bed thinking about sleep, I decided to get back up and wait until I was ready to collapse before trying again.
One thing occurred to me as I was climbing into bed last night (this morning, really) : over the past few weeks I've been following a pattern of lying on top of a bed for naps, and climbing in for the main sleep. This probably makes more of a psychological difference than I'd previously thought, as for the last couple of nights I've been climbing into bed on both occasions (cold nights, despite the daytime temperatures). I'll revert to the on/in distinction beginning with this evening's nap.
Following nap
Well at 3 hours that wasn't as bad. This followed the usual pattern; 90 minutes of sleep terminated by an alarm, followed by 90 minutes of 'natural' sleep.
At least I woke up prior to midnight, giving myself a real chance of following up with the scheduled bout of 4.5 hours' slumber. Ahhh.