That lack of adult company and the sense of isolation and loneliness is really soul destroying sometimes. I was there myself recently when i was ill and had no one to call on for support.
Suggestions i can offer would be:
Text some people in your contact list, it might not be adult company but at least you can converse with an adult. You have work colleagues you said, so find a reason to text them other than work reason that is!
I also now insist we go out for a walk on a sunday, doesn't have to be long but it gets us out and means i get a little free exercise. I also make the weekends one day chores day and the other day fun together day. So we go out if possible on fun day, or stay in and do an activity, board game, craft kit, watch dvd's with popcorn, read together, bowling, zoo, and lots of free and cheap days out to be found too. Not the whole time, but ten or fifteen minutes here and there through the day. It keeps me busy and makes me stop and have that quality time which can easily be missed. It all depends how old your children are i suppose as to what you and they want to do.
Perhaps invite a school friend of your kids over. At the very least the parent will have to stay for a few minutes when they drop them off, so invite them in for coffee. And again when they collect them the chance of another quick chat.
What about your neighbours, do you talk to them at all? If you don't then maybe it's time to introduce yourself.
Keep the radio on, listening to the banter and chatter can help make it feel like you've got another adult in the house, even if it's not a two way conversation.
Find some other single parents (you really won't be the only one at your children's school and maybe your work colleagues know some) and see if you can get to know them better. Then you can try a quick chat for support, or suggest taking the children out to the park so they can play and you can chat.
Find an activity you enjoy and join a local group - quilting, WI (you don't have to be an OAP to join!), ramblers, the PTA of the children's school... There's lots of things out there apparently. You will meet more people with something in common and have reason to call them up with quick questions about the next event, or ideas for forthcoming things etc.
Tell your collegues, family and friends you feel isolated and lonely, especially at the weekends. It might get some of them calling or suggesting meeting up at the weekend. And if it doesn't then you won't be any worse off than you are now really.
Maybe invite a couple of colleagues over for a dvd night, with their children or without! Ask them to bring a snack and whatever they'd like to drink and let the kids watch the tv while you chat in the kitchen. If the kids have their own tv then you can have the lounge instead.
And if all else fails come here for some company of sorts.
It's not easy i know, and it can seem like people just aren't interested or are too busy to make time or be friends. I've only started having more friends and people to call on recently, and that's because i joined a lone parent group but also because i've made myself start chatting more with other parents in the playground and when they say 'any time you need help just let me know' i now take up their offer, whereas before i used to think that's a nice thing to say but they don't mean it. Most of them do mean it i have now found.
Hope your weekend flies past.
take care
Gilly