Thanks, Plasma.
The garage said they'd have put 0w-30 in, which I gather (from another youtube video!) is less viscous at lower (cold engine) temperatures? So I got some of that, figuring it might be better not to mix it with 5w-30, though not sure if that matters? It's Valvoline, supposedly recommended for Peugoet and Citroens. £9.50 for 1 Litre from a tiny local motoring shop in a small town of 4000. Apparantly some petrol stations don't stock ANY motor oil nowadays - I tried one en route!
For the record, putting 250ml in brought it up from a shade below the minimum mark (as discussed/pictured above) about a quarter of the way to the max mark.
I guess more is better, from a lube point of view, so long as you don't go over the max. obviously? (Anticipates ribald comments...!)
Looking at my last service bill, I was charged for 5 litres in early summer. I'd be interested to know, from those HDi folk who change their own oil, how much they put in, altogether, and where that usually points to on the dipstick...?