I've managed to drive out to inspect the flooding at some of the most likely locations this afternoon. There's quite a bit of deep-ish floods (8-10 inches) after Terling (the mini-ford, normally dry, currently fast flowing, though the footbridge might just be reachable), after High Easter, and later on, back towards Witham either side of Ranks Green.
The Pincey Brook ford, after Hatfield Broad Oak is very deep, with the footbridge and its approaches under a lot of water. It might have subsided a bit to be useable tomorrow (it takes run-off from the airport which causes rapid rises in level) but the B road alternative described on the route sheet might be a wise move: essentially continue from HBO up to L @ the Four Ashes X at Takeley, follow the B1256 to Thremhall.
The gritters are out this evening, which doesn't look very promising. Plan B might be to reverse the route, taking the B road (Faulkbourne road) to Braintree, then the Dunmow B1256 to Rayne to pick up the Great Saling lane. With any luck the frost should have lifted by then ...