I fully sympathise with your situation, but there are certain aspects that you may need to consider. This isn't to defend an employer, I don't have all the facts in order to make a judgement one way or the other on that, just as with everything there are two sides to these issues.
Firstly, while it is difficult to do this, the issue of the work-related accident and you absence from work are two separate issues. I know they are linked, but procedurally, they are separate. The absence from work is managed and handled as any other absence irrespective of how it is caused. We may feel we are owed more consideration and flexibility, but this isn't always possible and an employer has to be careful what president they set. They have a policy and they have to follow it.
This may give the impression of an uncaring or inconsiderate employer, but they have to make sure they follow their systems.
Next is that there is only a limited amount of "protected leave" in any statute. That is maternity leave and annual leave. Everything else has no statute to back up proper procedure or protection, it comes down to what is contracted.
You say you have never seen a contract of employment and I have no reason to doubt that. It isn't that uncommon that employees who have been with their employer for a number of years weren't provided with a contract as we would today. However, this doesn't mean the policies and procedures don't apply. It might make it messy, but depending on certain circumstances they could still apply.
One question could be did you ever sign anything related to pay and conditions, it might have been included there. Additionally, if there is union recognition, the employer may have agreed a change to contracts or terms and conditions of employment through common agreement and a union vote. Lots and lots of potential permutations to say that the absence policy is valid.
At the moment, you have a note from your GP or specialist to say you aren't fit for work. The employer is within its right to seek a second opinion on this. I presume that because you are still absent, their doctor hasn't disagreed to a significant extent the decision of your GP. However, at every review stage or renewal of a sick note, you employer is within their rights to get that second opinion. So I would expect that if you reissue a sick cert, you will be asked to report to their doctor. It isn't unreasonable, though it might seem harsh and unfair, it is established within Labour Courts as being fair.
Again it might be hard to separate the cause of your injury and the absence, but injury and sickness are not protected leave. Most employers have a reasonable time limit of continuous (not continual) absence from work. At the end of that period they are within their rights to determine if you are fit to complete your contract of employment. Harsh, I know and especially in the circumstances, but in within the remit of the law they are two separate issues and leave following an accident is not protected.
So it is possible that your suspicions are true, but from an employer's prospective they cannot keep a job open indefinitely and have no obligation to do so. However, in your favour is if there are lighter duties that could be done, this may work for both sides and as you say could be discussed with the employer and yours/their doctor. In addition, in your favour is the aspect that some injuries are seen as a "disability" under equality legislation. As such, if you wish to return and there are other, lighter, duties available, then your employer may be required to make reasonable accommodation for you.
I'm not trying to preach or take sides on the issue, it's just that your employer does have to follow a certain set standard and procedure. In light of how you were injured which led to the absenteeism, it could seem harsh and unjust, but irrespective of that the policy has to be followed. It's not always the case that the employer is trying to get rid of you or harassing you.
The aspect of what caused the accident and where fault lies is, I'm afraid, irrelevant in terms of your absence from work. That’s a matter for the PIAB, or as seems likely now, the courts. I can't comment on that issue.