First from the HQ to the start -
Turn right and head north on North Hall Road for about a mile till you come to a T-junction with the B1383. Turn right and continue heading north on B1383, up through the village Newport, towards a roundabout with the B1052. The start is just before this roundabout (i.e. south of it).
Then the race itself -
Head north on the B1383, go past Audley End (stately home on the right), through the village of Littlebury (where the road bends to the right and then to the left), through the village of Great Chesterford (where the road does the same thing), up a long drag to Stump Cross roundabout. This should have marshalls. The first time you go there, you turn left on the B184 and then go straight over a series of three roundabouts in quick succession (these ones won't have marshalls). You are now heading north again on the A1301.
Next you come to the Sawston roundabout (which should also have marshalls). The first time you come here turn right (3rd exit) on the A505 and head north-east towards the A11. This is a busy and fast dual carriageway, but you don't really join it as such: you stay in the left hand lane all the time and your slip road which joins the A11 soon becomes a slip road immediately leaving the A11, taking you up to a big roundabout with the A1307 called 'Four Went Ways' (again, there will be marshalls here). You go right round this roundabout (4th exit) and start heading back where you came from. As before, stay in the left lane at all times and you've barely joined the A11 before you're on a slip road leaving it again and back onto the A505 heading south-west back towards the Sawston roundabout.
When you reach the Sawston roundabout (for the second time) you turn left and head south retracing your way over the series of unmarshalled roundabouts until you get to the Stump Cross roundabout (for the second time). This time you go straight over it (2nd exit) and head south-west on the B184 until you get to a roundabout just outside the village of Little Chesterford. Again, there should be a marshall here. Go right round it (4th exit) and head back where you came from all the way to Stump Cross yet again.
When you get to Stump Cross roundabout (for the third and final time!), turn left (1st exit) and retrace your route south on the B1383 down through Great Chesterford, Littlebury, past Audley End and down the hill to the finish line, just before (north of) the roundabout where you started from.
@JamieF
OK, the route:
First from the HQ to the start -
Turn right and head north on North Hall Road for about a mile till you come to a T-junction with the B1383. Turn right and continue heading north on B1383, up through the village Newport, towards a roundabout with the B1052. The start is just before this roundabout (i.e. south of it).
Then the race itself -
Head north on the B1383, go past Audley End (stately home on the right), through the village of Littlebury (where the road bends to the right and then to the left), through the village of Great Chesterford (where the road does the same thing), up a long drag to Stump Cross roundabout. This should have marshalls. The first time you go there, you turn left on the B184 and then go straight over a series of three roundabouts in quick succession (these ones won't have marshalls). You are now heading north again on the A1301.
Next you come to the Sawston roundabout (which should also have marshalls). The first time you come here turn right (3rd exit) on the A505 and head north-east towards the A11. This is a busy and fast dual carriageway, but you don't really join it as such: you stay in the left hand lane all the time and your slip road which joins the A11 soon becomes a slip road immediately leaving the A11, taking you up to a big roundabout with the A1307 called 'Four Went Ways' (again, there will be marshalls here). You go right round this roundabout (4th exit) and start heading back where you came from. As before, stay in the left lane at all times and you've barely joined the A11 before you're on a slip road leaving it again and back onto the A505 heading south-west back towards the Sawston roundabout.
When you reach the Sawston roundabout (for the second time) you turn left and head south retracing your way over the series of unmarshalled roundabouts until you get to the Stump Cross roundabout (for the second time). This time you go straight over it (2nd exit) and head south-west on the B184 until you get to a roundabout just outside the village of Little Chesterford. Again, there should be a marshall here. Go right round it (4th exit) and head back where you came from all the way to Stump Cross yet again.
When you get to Stump Cross roundabout (for the third and final time!), turn left (1st exit) and retrace your route south on the B1383 down through Great Chesterford, Littlebury, past Audley End and down the hill to the finish line, just before (north of) the roundabout where you started from.
Clear as mud? Someone has uploaded the map as well: http://www.mapmyride.com/gb/saffron-walden-eng/e1-25c-route-22251588