Tansy, I have already said a lot of what I have to say... But...
I think that if you are working at home, you absolutely MUST have peace and/or childcare.
I do understand your dilemma, because I was also self-employed when DS1 first started being difficult. I actually had to back out of one contract because he had been excluded from school... I really value my professional reputation, and that was a very big deal for me.
I also understand how hard it is to change the way you have responded to them, but you are going to have such a problem on your hands if you don't.
It will be totally impossible for you to enforce a PC ban while you are working, in exactly the same way that you could not enforce it if they were at school, or going to a childminder or their dad's. I think you will have to separate out your 'work time' from your 'home time' and enforce any/all sanctions during 'home time'. Other people may disagree, but my experience tells me this is the only workable solution.
I think you need to sit your DSes down and spell some things out. But you have to believe them yourself; if you don't, of course they will not believe you.
Introduce the no violence rule. As of today, there is no violence in your home. The past is history. From now on, no hurting/hitting will be allowed. At all. If anyone hurts anyone else, they should expect serious trouble. (BTW, this includes you too. I think you'll have to give up on forcing them into their rooms. It won't be possible for much longer anyway).
Tell them that any violence, however 'minor', will lead to an immediate withdrawal and an absolute minimum of 24 hours total equipment ban. No PCs, no TV, nothing. Don't tell them that you will not try to enforce this while you are working (because you need to keep things peaceful), but do add any time they cheat or break the ban on at the end of the ban, at a time when you can enforce it.
Tell them your work time is sacrosanct. Your work is what puts food in their mouths and pays for their PCs. If they behave, have not been violent and do not disturb you, you will allow them to amuse themselves (in any reasonable way they choose) for the 5 hrs/day you work. On the other hand, if they disturb your work in any way, they should expect serious trouble, AND you will make immediate arrangements for them to be cared for by someone else while you work. Emphasise that all the best holiday schemes are already full, so they will have to put up with what is left - or they may end up with a babysitter while you work elsewhere.
You may also like to offer a positive bribe reward - I think I would - something like If you behave, have not been violent and do not disturb me, you will get X at the end of the 5 hours... X could be cash, a trip out to do something they like, a favourite meal, etc... Rewards have always worked better with my DSes than punishments.
You have to mean all this. Don't say anything you don't mean, or make any threats you won't carry out. Ever.
If you are like me, you will be wondering why your DS1 is suddenly behaving like this. There will probably be several reasons, including habit, lack of consequences, hormones, lack/loss of self-control (common at this age) and/or anger. About the same age, my DS1 started to angry about the fact that his dad wasn't around... This could be a factor for your DS too...
You may not ever disentangle the reasons, but you still have to deal with the behaviour. You don't need me to tell you that you will have a serious problem if you don't...
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from me too. It's bloody hard work, this parenting malarkey!