Disclaimer: this is not legal advice but my personal views!
Under Data Protection legislation as the solicitors hadn't got your information directly from you , they were obliged to tell you where they had got it from. This is exactly what the legislation is designed to do - to make it transparent to individuals who has their information, where and why.
Your Parish Council is also required to be transparent about how it uses and holds the information that parishioners provide. If you made a subject access request to the PC it would similarly be obliged to tell you who it has shared your information with. Have you tried doing this? It might help clarify matters for you.
From the information you've provided it appears you believe that information you provided to the PC in order for it to exercise its statutory functions may have been accessed and used by the Clerk for other reasons. If what you're claiming is correct, that's a very serious matter but sadly not uncommon. Just because you can, it doesn't mean you should!
If you look on the ICO's website under 'action we've taken' you'll see many examples of criminal prosecutions against employees who have accessed information for unauthorised purposes. This month, the ICO took action against an employee of Stockport Homes who had inappropriately accessed records without any business reason for doing so. In March an employee at the Heart of England NHS Foundation Trust was prosecuted for accessing the medical records of people she knew without any business need. Last December a former doctor's surgery employee was prosecuted for accessing staff and patients records outside her role.
Therefore I'd echo the advice of others and suggest that you ask the ICO to investigate your concerns.