If you are coming from the M1,do as I did last week.
The signposted diversion is from j23 of the M25 north on the A1 to the A414,then east to the A10,south back to j25 of the M25.
I came off the M1 at j7/8,take the A414 to the A1,(about 5 miles),then you have to go north to the next junction to pick up the A414 again,east to the A10,then south to j25 of the M25.