Billie I would suggest you write to your employer factually and politely with chronological details of what happened. Talk about it in terms of " I wish to highlight my concerns about .... x y z".
Summarise any info relating to your pg, the lack of risk assessment, the protracted delays, any lack of action you experienced from them etc. Refer to any company policy you believe they committed to, but did not complied with.
Avoid at all costs any specific criticise of individuals, don't mention people's names. You can get round this by stating in the passive, eg "on xx March 2018, I was informed that xyz would be done, but it didn't happen".
Include a calculation of £value you were expecting. You are effectively stating it's a loss due to their inaction/negligence (but don't state it in those terms, of course - you need to stop short of a direct accusation.).
Write the letter as if you've taken advice from a solicitor. They could think you have!
This is exactly what you would have to do in preparation for a Tribunal Claim, to evidence how you highlighted your concerns.
Your letter is likely to trigger some response from them in terms of their justification. It may not be the answer you want to hear but in forcing them to state their position, it will indicate if they will tough it out.
Sending them a well written letter is far better investment of your energy that going through a Tribunal process with no real prospect of coming away with any financial gain (but significant costs!)