If you are looking for a mid summer, evening workout then you could do worse than Wakehurst Willow: a short 8k trail through the grounds of the Wakehurst estate, West Sussex.
The course is two laps of the beautiful grounds, is mainly trail and, for the most part is undulating. But, about a quarter way around the lap there is a challenging descent which you have little choice but to commit to. And what goes down must come up - there is an equally challenging climb! Both are great fun though and it is great training for hills and trails alike.
Being in the grounds there isn't a great deal of support other than the marshalls and those who have accompanied runners. You wouldn't notice it for the encouragement given and noise created though. It has a really friendly atmosphere and a great run.
One last thing: it's tiny! Approximately 350 runners get the privilege so be quick to sign up!