No marked trails at Stanmer – they’re ‘tolerated’ rather than official.
Best bet is to go over to brightonmtb and post on their forum to see if anyone’s able to guide you. failing that have a look on strava – most of the trails are segments on there now (although translating the gps tracks into trails on the ground can be tricky).
Stanmer to Ditchling Beacon is pretty easy either on or off road – there’s bridleway headign North out of stanmer that (via a long steep climb) heads straight up to the sdw and it’s only a couple of hills from there to the Dyke.
Do take a map, though, just to be sure.
There’s a YHA at Truleigh Hill on the SDW about 2 miles west of the Dyke – given the weather it might be worth heading there for the night?
I rode through the travellers camped on the dyke to hove cycle track a couple fo days ago and other than a few mad dogs yapping at my ankles I wasn’t actually pelted with stones this time which is an improvement over previous experiences. I’d cut on to the road alongside for the bit where you see the caravans if I were you.