This is a significant issue for me resulting in an outcome that feels deeply unfair and is far from ideal.
There is one very simple question did your seek specific advise in this rergard to get sorted, if you engaged and paid someone to sort then i would like to hope the pi insurance of the firm involved would cover it.
If you had informal chit chat and simply presumed everything would be ok without enagaging (and presumably paying someone ) someone to sort on your behalf then i would kind of expect this situation to arise.
Your chosen tax saving route is at the advanced level of things (depsite the fact it may be simple to complete forms - the skill is in knowing what to do)
So starter for 10 question did you engage and pay soemone to fully sort this issue - if you did have you checked with taht firms pi ?
in summary form 17 is something that needs be looked at at the very top level - its you signing on the dotted line - imho its 100% up to you to engage whoever to makke sure that was fully sorted on your behalf.