Yes they can trace you mate, if you don't tell them and they find out then it's a non-disclosure of what they call a material fact.

They can find you by doing a 'CUE' check, Claims Underwriting Exchange, basically a central database where all accidents are recorded.

Tell the truth, it's easier in the long run, your 3 years ncd is still valid as it was on another policy but you do have to disclose the claim

HTH