I did a little bit of reading on washing fleeces on Ravelry and it seems many have good results with soaking the fleece in cold water before getting into the hot wash part.. So yesterday I filled a large tub/bucket with cold water and put the white fleec in overnight.
It was surprising how much dirt came out just from the soak..
We (because of course I had a helper ;) ) then washed it in two batches in hot water - two 'washes' and one rinse for each batch.
I then placed the fleece into lingerie bags to spin it out in the washing machine and it is now laid out to dry on a clothes horse outside.. I will move it into the shed to dry out of the weather, though. Once it is dry I imagine the fun begins - combing it to get rid of the grass/seeds/burs.. and there is so.much.of.it right through the whole fleece.