Many thanks to all that have taken the time to reply to my post.
I am no longer in the business as of today.
Southpaw as expected provided extremely valid comments.
In terms of a subject access request, it wasn't about bringing into disrepute my line manager. It was genuinely about having some understanding of their reason to accept my resignation and refusing to accept my resignation withdrawal.
I really wanted disclosure, to see where I went wrong.
However, having been put in a situation where my line managers may have been frustrated by, my following company procedures, I compiled a whistle blowing document to the top people. And yes before you all shout and say this is an attempt to get my job back...I am not interested in being reinstated as it would be conceived as blackmail. Yes I would very much like my job back, but only on the value of what I can provide the company. My line manager has no engineering skills and is a regional maintenance specialist. Her line manager has also no engineering skills.
Anyway, in essence I don't expect any responses, since you have all done more than enough and I thank you all for that.