This subject has been on here so many times, the most popular combo seems to be pig oil and sulfer applied to the legs once or twice a week and the mud brushed off when it dries.
To TREAT it then you will need something antibacterial/antiseptic. I use Equimins winter leg scrub to wash legs with (very mild, and you don't wash it off) and they also make a mud fever lotion. Both with added silver which has great antibacterial properties. Make sure legs are clean & dry (I would be tempted to keep in until its cleared). Then to PREVENT it happening again, pig oil is very good, but any oil based barrier will work well (pig oil is just the cheapest). I don't wash legs every days as I think this makes it worse, but obviously depends how muddy your fields are
Flammazine should work as it is anti bacterial, however you need to get it right down into where the bacteria are breeding. This meanns getting the scabs off. To do this you need to wash with anti bacterial scrub and then pick them of. If they are realy thick and crusty you can slaver the legs in something moisturising and wrap them in cling film and a stable bandage for an hour or so toihelp shift them. If you have a feathery horse these need to come off too (sadly). Once the scabs are off you must dry thoroughly and then put anti bacterial cream on. I put anti bio cream on twice daily and find I only then have to descab 2 -3 times a week. About 2 weeks on we were clear so then moved onto barrier methods to prevent it coming back. Another thing to remember is not to cross contaminate so use different brushes/sponges for legs that have got it.
I have also found Gold Label mud guard very effective, if the mud fever hasnt got hold then it will treat it too, if you have a lot of scabs you need to get rid of them first. Having had a a horse with 4 white legs Ive been through every lotion and potion ! I used to use baby oil to soft the scabs and then gentle clean them and treat with Protocon - then once I was on top of it used to apply the mud guard every day.
the muddy marvel range is very good! de scab lotiion, muddy marvel barrier cream and mm 7 day protection (or similar labels too late to go and look now!!) Get your vet to make you a mud fever cream ... aqueous cream plus anti biotic plus steroid, workd brilliantly!!