Tax on credit union DIRT and interest

I have been very lax with this issue and now it is coming back to haunt me.
I left about 10k sitting in a CU account for the last 7 years in a regular share account. I did nothing with it and stupidly was not thinking about the tax liability on the interest/dividends accruing.

I now need to use the cash as I am building a house. But I want to settle what I need to with revenue.

I feel really really stupid I left this go. My only excuse is that I never did have to do any tax returns as I my employer did it for me and had no other extra earnings.

I am taxed on the higher rate.
Anyone know how I would go about this? Will I have to put in a form for each year. And what am I liable for 42%?

DIRT is not relevant here as far as I know - income tax may be. In the past all CU dividend income was assessable for income tax at the individual's marginal rate and it was up to the member to declare this. Things have changed in more recent years.

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You need to figure out what your precise liabilities are likely to be.