I'm a bit late checking in because I've been cooking dinner, but I've carried out the same sunset observations as Paddy this evening (Good idea BTW)
Now clearly this isn't completely scientific because cloud cover would vary but I think it gives a reasonable representation.
Official sunset here 18:21 and it had clouded over a bit from earlier.
18:21 (sunset) still light out and good visibility
18:31 (sunset + 10 minutes) starting to get dim, but I'd still classify it as light and no street lights on
18:41 (sunset + 20 minutes) Light fading and street lights on but visibility still good enough to see to the end of the garden and determine shapes and colours. Although orange street lamps are lit there's enough ambient light that the orange lighting isn't interfering with colour perception. I'd classify this as dusk
18:51 (sunset + 30 minutes) Still some light in the sky visible between the clouds but definite reduced visibility outdoors. Orange street lamps are starting to skew red coloured vehicles in the street. I would classify it as dark at this point in time
Sunset in Machynlleth on the 1st October was 18.52, so by your reckoning it would not have been dark until 19.17 (probably a minute or so later, as the period of dusk shortens as the days get shorter).
Sorry Paddy, got a little confused with your timings and need to re-read them but based on my observations and (I think) yours I'd draw the same conclusion as veggiefan. I reckon that at 19:00 in Machynlleth (8 minutes after official sunset) it would not have been dark although with heavy cloud cover the light would be starting to fade.
EDIT : Actually just read Paddy's posts again and I think I misinterpreted.
Did you say that it started to get dark with you 10 minutes before official sunset?
Out of interest what was the weather doing with you and were you judging how dark it was from indoors or outdoors?
I did notice it getting a little darker indoors in the approach to sunset, but I actually stood outside on the deck to observe what light levels were outdoors.