Well, thankyou for all your comments! I took everything on board and after long hard look at the budget, we ended up building this version:
The first thing is that after totting up the price of sheeted hurdles and guillotine gates, the race has gone completely, and we now just have two small pens and one larger one. This wasn't cheap either of course, but since the only costs were gates, posts and gravel, it wasn't actually as much as you might think!
The sheep are herded in uphill through either of the 16' gates, and from there the internal 10' gate can be used to direct them into one of the smaller pens. The idea is that I can then treat sheep in small batches in the smallest pen before letting them go individually either back into one of the other pens or out into the field as appropriate (let's face it, how often do you need to shed such a small flock anyway!). My logic is that if we need to 'sort' them further, it should be easy to add another temporary pen or two around the periphery using hurdles.
The gates of the wee square pen open both ways, and if you open them inwards, you can use them to let a single sheep out quite easily, and if the wrong one tries to escape without permission, the gate will just bang shut away from them.
Of course some sheep are more trusting.....
........ than others
(Don't go in Alice - it's a trap!
)
But I hope they'll come round to the idea eventually!
So thankyou all for your comments - I'm pretty pleased with what we've ended up with. The suggestion of putting hardcore down was also great (these stones are maybe a bit big, but were half the price for some reason), and should stop everything from becoming a muddy mess.
Thanks again all!