Sooooo... the lesson for today is: 'Gravel. It's skiddier than you'd imagine'
Thanks to the encouragement from this thread and all fired up with enthusiasm, out for the evening 'round the block' and aiming for anything close to an average of anything near 12mph, I was bravely holding off putting the brakes on too soon as I went down the Big Hill (admitting to myself that 7mph is a little bit slow going downwards, even by my standards). At 14mph I cracked and decided to gently start applying the brakes, but stupidly hadn't twigged until today that if you keep away from the verges (drain covers; potholes; nettles and brambles) and if the road is narrow, you end up off the part of the road that cars use all the time and drift into the death trap that's known as the 'middle' where all the gravel gets flicked to and, I kid you not, it's a slippy beggar to deal with. Muy stupido, I know, and from now on I'll keep to the less treacherous parts of the lanes and deal with the occasional nettle sting.
There was also a bit of an adventure with a careering horse and a double-trailer timber lorry driven by a lunatic, but overall a nice enough evening to keep me fired up and ready to head out again tomorrow.. Average speed: 11.8mph. Average terror: 7/10