Ok, I'll try to summarise the last several months of my experience in a few paragraphs.

I've been at my company for a little over 18 months. At first I entered as a junior developer, since I only had 2 years experience. After around 6 months, my line-manager (call him Steve), offered me a senior position, as they had interviewed many people for it and had not filled it. I told him I would accept as long as he promised to train me up, as I felt I was not suitably qualified at the time. Until this point, myself, Steve and a couple of the others were all friends, having a good time at work and doing social things outside the office.

Then everything changed. I don't know why, but I think Steve regretted giving me the position. He began speaking to me in unacceptable ways, i.e. swearing, barking orders at me and criticising everything I did, often verging on basically calling me stupid. The training he provided was basically writing code in front of me really fast. I didn't pick much up this way, but could tell he was trying to teach, so felt bad to say anything.

I kept a log of all of this and eventually, after 6 months of it, went to HR. I made a formal complaint about him and submitted details of exactly what he had said to me with times and dates and even a couple of witness' within the team.

HR investigated the case for several weeks, (during which time I had no work, as they said I could not work with Steve during the investigation, so I had nothing to do for 10 weeks, but still getting paid).

The outcome is that HR are doing nothing and that none of my claims can be upheld. Basically, he lied about everything, saying he didn't speak to me like that and shockingly, my colleagues lied for him as well, saying they never heard anything.

So now I'm in a position where I have no work, and he is refusing to accept meeting invites from me now the investigation is over to try and get some work and basically just move forward.

How can I salvage/repair this situation and co-worker relationship?

Before anyone suggests it, yes my line-manager has a manager of his own, unfortunately they are very good friends outside of work, so I've tried that route with no success.

Given the limited information here, my conclusion is that you drew the short stick on an internal power play.

Steve definitely set you up to fail as a senior dev. He promoted you to the position knowing you lack the hard skills necessary to do the job, he did not properly train you, and then he used your lack of skills to motivate his decision to not work with you. I don't know what his motives are, but the way you tell the story, it's pretty clear to me he was harassing you so he would get rid of you.

Your failing lays in not realising you were being set up and not taking action in a timely manner. The second Steve started "training" you and you couldn't understand what he was doing is when you should have told (in writing) your manager or Steve's manager that you are not receiving proper training and you cannot be expected to complete your tasks.
Furthermore, Steve's frustration with you grew as he probably saw you are thick skinned and his insults/harassment aren't working.

Your next mistake is taking things up with HR. HR is not there to protect you from Steve's bullying, but to protect the company's interest. It was HR's conclusion that Steve's work is more important than yours, so their internal investigation came out blank. Not that it matters anyway, you are now flagged as a problem employee. Next time someone has to leave, guess whose turn it will be.

A common way to convince an employee to leave is to take away all his usual tasks and replace them with what basically amounts to worthless junk. We had a case in my country where an employee got relegated to shredding paper so she would leave the company. To me it looks like your employer is trying to get rid of you by not giving you tasks and waiting for you to get bored and leave. Or fire you whenever it suits them because you're not being productive or some made-up reason so you won't sue them for unlawful termination.

So in conclusion, Steve won his little game, you are working in a dysfunctional company which is an unfair employer that only cares about Steve and Steve's gang's happiness and is willing to sideline anyone Steve doesn't like. And Steve HATES you.

Your choices are rather limited.

1: You make it up to Steve. Apparently his happiness matters in the company more than facts. Make Steve happy and you'll be back to your old tasks in no time.

2: Realise you are wasting your time in this company and start looking for a new job.

3: Continue in the current situation and wait to become the equivalent of a human paper shredder. Until they find a good reason to fire you.

You launched a formal complaint complete with witnesses and documentation which was investigated for 10 weeks while you were paid for doing nothing, and then your complaints were found to be groundless, even your witnesses did not back you up.

Is this co-worker relationship salvageable?

No. You put multiple people into a difficult position. Things will never return to how they were. More constructive to worry about being forced out altogether. You should have spent the 10 weeks job searching, suggest you get onto that asap.

Then everything changed. I don't know why, but I think Steve regretted
giving me the position.

I'm guessing there's more to this than just a sudden change of attitude. It's unfortunate you didn't talk to Steve to find out why.

How can I salvage/repair this situation and co-worker relationship?

Before anyone suggests it, yes my line-manager has a manager of his
own, unfortunately they are very good friends outside of work, so I've
tried that route with no success.

You could try talking with him and seeing if you can determine why he changed. Perhaps you have something to apologize for here. If so, it would be a good time to do that now. But your relationship may already be beyond the point of repair.

It appears that instead of talking to him directly, you chose to create a 6 month log and then took it to HR as a formal complaint. And apparently, you also went over his head and tried to complain to his boss.

It's not clear why you chose that route first rather than trying to repair the relationship, but what's done is done.

You might need to watch your back now, and start thinking about finding a new job or at least a transfer to work with another manager.